FREQUENTLY ASKED QUESTIONS
How important is it to change oil regularly? What are the implications of failing to do so?
It is only really important to change oil regularly if the bike or car covers a low annual mileage made up of slow, short runs. This is being cruel to the oil and the engine! The oil, regardless of its quality, gets full of fuel and water vapour, and never gets the chance to evaporate it all off with a long fast run. The consequences are corrosion, ring and bore wear, and gear tooth pitting. It is essential to do a change at least once a year, even if the recommended mileage hasn’t been covered. On the other hand, if you eat up the miles on long blasts the engine and its oil will love it, so with a top-quality oil it is OK to cheat a little on oil drain periods.
What are the likely consequences of using poor-quality oil?
Usually, these are fairly long term, except in racing. Think of the oil as a liquid component, and poor oil as a cheap pattern spare. In a touring bike or an ordinary family saloon, long-term reliability and performance retention (i.e. acceleration figures below new spec., fuel and oil consumption above) are the casualties. In a high performance car or racing bike, the effects can be more immediate and catastrophic. I recently saw a cylinder block from a racing engine. All 4 bores were scored from top to bottom on the thrust sides, and all 4 pistons ruined, needless to say. The whole casting had to be scrapped. After spending a lot of money on race preparation, the owner had saved a few quid by using a cheap 10W/40 ‘workshop’ oil, probably API SG or lower, and the used oil had sheared down to SAE 30. Now this is the really crazy part: it was the second engine he’d ruined on the same oil!
What are the most important substances added to the refined base oils and what do they do?
In the Dark Ages both bikes and cars used blends of refined mineral oils ‘straight’, with nothing added. The trouble was, even in the slow-revving engines of 80 years ago the oil didn’t last very long, and the engines didn’t either. Black sludge and corrosion were the killers, and both were tackled in the 1950s with detergent and antioxidant chemicals. The detergents washed the carbon from fuel combustion off the bores and out of the ring grooves, and at the same time reduced bore and piston ring corrosion. The antioxidants stopped the oil reacting with oxygen in the air, which cut acid sludge formation which in turn reduced corrosion and oilway blockages. Some antioxidants had the useful side-effect of reducing wear as well. This added up to longer oil and engine life, both improving about three times. (Straight oil had to be changed every 1000miles, and even lightly-stressed engines running on it were ready for a full overhaul at 15-20,000.) OK, I admit there were design and metallurgical improvements, but they needed that vital ‘liquid component’ to be fully effective. Later came dispersant compounds which held the carbon as tiny particles in the oil which didn’t settle out anywhere, and slipped through the oil filter as if it wasn’t there.(Solid bits in well-used modern oil are about 1/1000mm across; the pores in an oil filter are at least 15 times bigger.)
The other big problem with oil used to be cold starting. It was usual to have SAE 20 Winter or ‘W’ grades, and SAE 30 or 40 Summer grades, and even the so-called Winter types would defeat the starter in serious cold weather. Unfortunately, oil is very thick when it’s cold, and very thin when it’s hot. To have an oil thick enough to look after a hard working engine, you had to use a grade which was too thick when it was cold. The answer was (and is) multigrade! What was needed was an oil that behaved like a 20 ‘W’ grade in the cold, but only thinned down to a SAE 40 or 50 when really hot; yes, 20W/50! This can be done by mixing thin oil with thick polymers based on plastics and synthetic rubbers; these don’t do much in the cold, but as the oil warms up they unwind and thicken it up to some extent. The oil still thins down, but not as quickly as a polymer-free or monograde type. Multigrades started to catch on around 1960, but these pioneer types were easily ruined by mechanical shear effects, more so in gearboxes than engines. These days the better quality polymers resist shear even in combined engine/transmissions, so it is essential to use good quality shear-resistant types in a motorcycle, which gives its oil a hard time in both engine and gearbox.
Incidentally, there are large amounts of these additives and polymers in there, it’s not just ‘a little bit of this, a little bit of that’! A good quality mineral 10W/40 can be 80% base 20% additive chemistry, and guess which is the expensive ingredient!
How does oil work? What gives it its lubricating properties? How does it 'cling on' to surfaces?
A plain bearing such as a main or big end, when spinning fast, is ‘floating’ on a relatively thick film of oil. The metal surfaces literally do not touch. The high velocity drives a wedge of oil between the two surfaces, and the oil film supports the load, just like a water skier skimming over that very thin lubricant, water. But, when the engine slows down and stops the bearing shells drop through the film and touch the crankpins, just as the skier sinks in up to his neck when he lets go of the rope. It is where there is metal to metal contact that lubrication, that is something to reduce wear and seizure, is needed. On gear teeth, valve components, and piston rings at top or bottom dead centre, there is no high speed rotation to generate ‘wedge’ support, so the oil films are very thin, and some metal contact is inevitable. Some fluids, even if they look thick and oily, are completely hopeless! Very pure mineral oils, and some synthetics, fall into this group. They depend entirely on chemical load-carrying compounds which react with metal at high pressures and temperatures to provide very thin protective films which prevent micro-welds where metal surfaces come into contact. Detergent and antioxidant chemicals often double up as anti-wear agents. The odd ones out are esters. These are attracted to metal by electrostatic forces and cling on when surfaces are forced into contact.
Does oil have to be warm to do its job properly? Is it important to warm up your engine before driving at speed?
Yes, it does have to be at least warm, and preferably hot. Most people find metal at 60ºC too hot to touch, yet 60ºC is too cold for oil in an engine that’s going flat-out. The best approach is to use a good 10W/40 or even a 5W/40, and take it easy for the first couple of miles, especially in very cold weather. For racing, a really good warm-up is essential, except perhaps with special 0W/20 low-drag race oils. The trouble is, oil pumps are very good at pushing oil out at 60PSI, but unfortunately there is only 14PSI (atmospheric pressure) pushing it in! (Even less at high altitude.) So it’s easy for an oil pump to pull voids or pockets of vacuum in the oil if it doesn’t flow fast enough into the oil pickup. This ‘cavitation’ obviously reduces the amount of oil the pump can deliver. Also, in high-speed bearings the oil can be too thick to keep up with the high rubbing speeds reached in modern engines so the ‘wedge’ or hydrodynamic’ effect breaks down. I know it goes against common sense (whatever that is) but the faster a bearing is turning the thinner the oil should be. (A 4cm. diameter main bearing is rubbing its shells at 56 MPH at 12,000RPM! So to avoid cavitation the oil needs to be 10cSt or less, which is SAE 30 if the oil happens to be at 100ºC, or SAE 40 if it’s at 110ºC.)
How does a high-performance oil allow the motor to produce more power?
An engine wastes fuel energy in several ways, and most of them are due to the laws of thermodynamics, which is another way of saying you can’t do much about it. But up to 6% of engine output is lost due to oil drag, made up of pumping losses and viscous drag between moving components. The transmission is included in this in most motorcycles. Provided wear and friction are kept down, there are real gains to be made by using a ‘tough’ but low viscosity oil. Surprisingly, frictional losses are low, down at 3% or less even with conventional oils, so there are few gains to be made here.
I have actually seen this extra power output on the dyno! A very experienced operator in Peterboro who does a lot of test work for Lord Emap used his own-year-old Independent tests carried out using a Honda Blackbird backed up this theory. The first power run was made using a good quality 15W/50 high-ester synthetic engine oil gave 128BHP. The oil was then changed to a 5W40 high ester synthetic. (So it wasn’t an unfair comparison with a cheap £^ &* 15W/50!) This time, results were 131.6BHP with a corresponding torque increase. Finally race 0W/20 special synthetic oil was used and 134.4BHP appeared! The results of these tests have since been bourn out in actual race conditions; and, with the right chemistry to look after the engine and transmission internals, there’s no down side of increased wear.
What are (or can be) the main differences between oils of the same type, i.e. what's the difference between a 'good' and a 'bad' oil?
It all comes down to honesty really…..so beware! A good oil is what it claims to be on the can. If it’s a 10W/40? Does it really pass the cold test at -25ºC? Quite a few that have been tested do not. There is usually an API spec quoted, such as API SH or SL. These are car-based specifications, but a good basic quality guide. If absent, leave it on the shelf, and avoid lawyer-speak, i.e. ‘Meets the requirements of….’ or ‘recommended (by whom?) for use in….’. For motorcycles the JASO MA or MA2 spec is a good sign, because this Honda/Kawasaki/Yamaha/Suzuki-sponsored series of tests is entirely motorcycle-orientated, and includes a clutch slip test, a shear-stability test, and a high-temperature/high shear viscosity minimum permitted level. The ‘big four’ were pushed into introducing this series of tests back in the late 1990s by the poor quality of some oils. Also there is the ‘synthetic’ minefield to be considered. Provided the price hasn’t been pushed up by shipping an average oil 5000miles from the West coast of the USA, you get what you pay for. In general the best oils are made in the more developed European countries, but remember low price buys the cheap ‘modified mineral’ synthetic, with a poor multigrade polymer. As is so often the case, quality follows cost.
Please can you explain the grading system? What is meant by the weight of an oil? What does 10W/40 mean for example?
Weight means viscosity, or resistance to flow. Water and paraffin flow very easily, so they are low or light viscosity. Golden syrup or 140 gear oil do not come out of the can so easily, so they are high or heavy viscosity. Especially with oils, temperature is very, very important. An oil which looks ‘heavy’ at 20ºC will be very ‘light’ at 100ºC. People sometimes say, ‘I drained the oil when the engine was hot and it ran out like water…’ so I say, ‘Good! It’s supposed to be like that!’ The American Society of Automotive Engineers (SAE) ratings cover cold start viscosities and ‘up and running’ viscosities. There are two sets of standards, the ‘Winter’ (W) ratings, and the 100ºC standard ratings. (‘W’ does not, repeat not, mean ‘weight’!) So a 10W/40 oil has to pass a 10W cold viscosity test at -25ºC, and a SAE 40 test at 100ºC. In an oil lab there will be a refrigerated viscosity measuring device for the ‘W’ tests and another at 100ºC for the standard SAE tests. There are 6 ‘W’ ratings from the difficult 0W at -35ºC to the dead easy to pass 25W at -10ºC, occasionally used in India for example! The whole point of these Winter ratings is to assist cold starts, to get the oil circulating quickly, and to avoid power and fuel-wasting drag as the engine warms up. Once it is warmed up, the 100ºC ratings count. There are 5 of these, 20, 30, 40, 50, and 60.
Viscosity is measured in standard units called ‘Centistokes or cSt’, named after a Victorian engineer, Sir George Stokes, (who used to time ball bearings as they sank through oil to measure it’s viscosity). SAE 30 for example is from 9.3cSt to 12.5cSt, and SAE 40 follows on at 12.5cSt to 16.3cSt, although most SAE 40 oils are in the middle at about 14cSt. Now this is something most people and some engine builders don’t realise: engines do not know what grade of oil they’re running on. They’re not clever enough! So an engine filled with 10W/40 will be running on a viscosity of 14cSt at 100ºC, but with a sump temperature of 90ºC its seeing a viscosity of 18cSt, so as far as the engine is concerned it’s running on SAE 50. Likewise, at 110ºC, the viscosity is down to 11cSt so the engine thinks it’s on a SAE 30! (Which is preferable.) The lesson is, do not use power and fuel-wasting thick oils in cool climates. A decent 10W/40 or even thinner is perfectly OK unless the engine has been built with wide clearances and a slow oil pump.
Radical race cars use 1300cc Suzuki Hyabusas engines and work them very hard. They use a 15W/50 oil with a high ester content, but that’s OK because they see oil temps around 130ºC! (No problem for a really good quality oil or the engine, but they do fit special oil seals.) At 130ºC the true viscosity is 10cSt, so the engine thinks it’s on a thin SAE 30, which keeps it happy.
What are the main differences between 2 and 4-stroke oil? Why does 2-stroke oil have to be mixed with fuel?
2-stroke oil has a very short working life, straight in and out, and it gets burnt. The 2-stroke engine doesn’t have a sump full of oil and the bearings are all rollers, so there’s hardly any oil drag, hence no need for multigrades. Long term stability is obviously not a problem! But, 2-stroke must burn off without leaving any plug-fouling or detonation-initiating deposits. The detergent and anti-wear additives used in 4-stroke oil leave hard white ash behind when they burn, just what you do not need in a 2-stroke. So 2-stroke oils use low-ash detergents and dispersants, and the better types use ester synthetics to act as anti-wear compounds. With current environmental concerns, smoke is a sensitive issue, so most ‘road’ 2-stroke oils are now low smoke, which requires yet another type of synthetic base designed to burn off invisibly. For some rather basic but very high-revving air-cooled racing 2-strokes there’s still some sense in using blends with that marvellous anti-seize liquid, castor oil!
Due to crankcase induction and compression, the classical 2-stroke obviously cannot have an oil-filled sump, so the only way to keep an oil film on anything was to add oil to the fuel, or inject oil into the crankcase space where it could mix with the fuel vapour. There are now some engines where the fuel and oil are injected separately, but the oil is still burnt.
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