Car vs bike oil - some facts

[weasley]

In this forum and hundreds of others like it the eternal “which oil should I use?” question gets asked over and over promoting the usual car vs bike oil debates, arguments, mis-truths and myths. I wrote this originally as a response to a particular post but thought I’d actually make it a post in its own right.

So, for a bike, which oil is right? As with many questions, the answer is “it depends”. In most cases though, having some kind of oil is better than having none. Using an oil designed and developed specifically for your application will generally gain you the last fractions of performance, protection or efficiency but there aren't many situations where the "wrong" oil will catastrophically wreck an engine.

Most modern bike engines use the same oil for the engine, gearbox and clutch. Yes, there are exceptions (eg Ducati’s dry clutch, BMW’s separate gear oil, 2 strokes and numerous others) however, for the sake of simplicity, I’ll talk about ‘bikes’ as being the majority that use the 3-way oil system.

Engines, clutches and gearboxes have separate and sometimes conflicting requirements from an oil. For example, an engine oil is required to protect mostly sliding contacts whilst not absorbing too much energy – in other words they need to allow the engine to work efficiently. Reducing friction is the key to this.

Gearboxes have a different requirement – the contact is a more rolling/sliding mechanism, which demands different extreme pressure protection but again reduced friction is a bonus to help reduce power losses.

Clutches needs something different again. They need friction. Specifically, they need predictable, progressive friction that changes consistently with applied pressure. Friction curves need to be a certain shape, otherwise you can get snatchiness or slipping. Ultimately, when the clutch is closed you need total power transfer and no slipping. It’s worth noting that clutch slip isn’t just about when the engine revs up but the speed doesn’t – there can be a degree of clutch slip without you really noticing it. As each piston fires it sends a ‘jolt’ through the flywheel and clutch – if not set up correctly, this can cause a micro-slip in the clutch. Add these up and you can get a clutch spinning at 99, 98, 97% of the flywheel speed, thus losing you power and speed. Finally, different clutch friction surfaces need different oil characteristics to get the right friction curve.

Of course in a car, the engine oil just does the engine. In order to chase every last drop of efficiency certain additives can be used to reduce friction. Note, this is reduce friction, not reduce viscosity. They are not the same thing. Friction reducing additives are known collectively as friction modifiers (FMs).

We all “know” that oils with FMs are bad for bike clutches, but what we don’t know is which oils contain them. In fact FMs may not always be bad for a clutch, since FMs are generally targeted at specific surface types within an engine, and these are unlikely to be the same as a clutch surface, so an FM may just not interact with a clutch (and therefore not bother it). However, in an engine oil, there are numerous additives that are “surface active” (meaning they bond or react with a surface) such as detergents, anti-wear additives, anti-corrosion additives, even some base oils. In a pure engine oil, the selection and mix of these additives can be focussed on those that deliver the lowest friction whilst also doing their day job. This cocktail of chemicals may not be the best solution for a clutch though, which is where things can go awry.

Which oils have FMs in? There’s no list of ingredients on the can and safety data sheets only contain basic information on certain ingredients, which won’t help you. FMs tend to be organic molecules which don’t show up in traditional oil analysis (like zinc, phosphorus etc do). You can get clues from the specifications though. As has been previously pointed out, any oil with an "energy conserving" API specification may contain a FM. However, in Europe the API specs are largely redundant and so you either won’t find it on a car oil, or if you do it will be accompanied by other specs; ACEA and various OEM specifications are more prevalent (on car oils). There are essentially no useful bike specifications, the most common being the JASO T903 spec which really only covers clutch slip, not full drivetrain protection.

ACEA specs (for cars) look like this: ACEA Ax/Bx or ACEA Cx (where x is a number). A and B specs are always paired up; C specs can stand alone or (at the moment) go along with certain A/B specs. A/B specs are 'full SAPS' and contain additives which can upset certain catalysts or DPFs. C specs are 'low SAPS', which means they have lower amounts of the DPF-blocking additives. A is for petrol engines, B is for diesel. C covers both.

Now, there are certain ACEA specs that allow for improved fuel economy, but typically through allowing a lower HTHS viscosity (high temperature, high shear). ACEA A1/B1, ACEA A5/B5, ACEA C1 and ACEA C2 all promote fuel economy and have a lower minimum HTHS requirement. Note - no mention of FMs - it is possible that any oil has an FM in it, or doesn't.

Further consequences of the single oil system in a bike is that the clutch and the gearbox put a high shear stress on the oil; this can break up long molecules into smaller ones. The most vulnerable molecules are the polymers that are added to make an oil a multigrade. Their job is to stop the oil getting too thin when it’s hot. When they get chopped up they can’t do this job any more so the oil will thin down in use. This can lead to reduced oil film thickness and also increased oil consumption. There are many different types of polymer to do this job and some are more shear-stable than others – better bike oils will tend to use the more stable (but more expensive) versions, whereas engine oils might not.

Another difference with bikes vs cars is the amount of oil per power unit. A bike has less than 3 litres of oil to deal with up to 180 bhp and 18,000 rpm (if Yamaha can be believed!). Cars tend to have larger sumps, which means more oil to do the job, giving each unit of oil an easier time. Cooling systems on bikes are often quite limited too, with smaller, lighter radiators and water pumps. Hotter oil is thinner, oxidises faster and evaporates more.

The only way to be sure that the oil you use is suitable for your bike is to use a bike oil of the recommended specification. Of course there are numerous car oils, truck oils, tractor oils and myriad other oils that have gone into bikes over the years (accidentally or deliberately) and caused no problems; experience can count for something but what worked for one person in their situation won’t always work for you in yours.

Me? I use a bike oil, a semi-synthetic 10W-40, usually either Silkolene or Castrol. Would I use car oil in my bike? In a pinch, probably yes, but not if I could get hold of a bike oil.

How do I know this stuff? It’s my job to know this stuff and has been for 19 years; engine oil product development, technical support, analysis and technical training. I don’t know it all, by no means, but I’ve picked a bit up over the years.
____________________
Yamaha XJ600 | Yamaha YZF600R Thundercat | KTM 990 SMT | BMW F900XR TE

[devojunior]

so i can use an oil for a car as long as i know it has no fms in there. cheers sausage chops probably cleared it up for loads of people just nobody can be bother to do the research.

[weasley]

I'd be interested how you can find out that an oil has no FM in it. And even if it did have, it might be OK, or it might not. That's kinda the gist of the post!
____________________
Yamaha XJ600 | Yamaha YZF600R Thundercat | KTM 990 SMT | BMW F900XR TE

[whitedevil]

So the conclusion is its best to use a bike oil in a bike. Who'd a thought.
____________________
GPZ500 sold ~ CBR600FS-2 sold ~ ZX6R sold ~ Street Triple R

[Frost]

Yes but i always thought that running a bike on expensive bike oil and not changing it for ages due to the cost was worse for your engine than running it on cheap car oil but changing it regularly. But i've only ever owned knackered old bikes so neither would probably make much difference

[iooi]

[temeluchus]

If I was a rich barsteward I would be running krytox in my motor.
____________________
Some shite cruiser. Now with guns and FREEDOM!

[cb1rocket]

I have ran castrol GTX in almost every bike I have owned. Its a semi synthetic base oil and comes in 10w 40 which is usually the common grade. If it was to be an R1 etc then I would fork out more for a good brand but not a rip off version with loads of nonsense blah blah advertising on the bottle

[Blau Zedong]

I use fully synth in my GSXR but just halfrauds crap and change it evey 4K. While it would probably be fine on semi synth I just have it hammered into my psyche that fully synth is what should go in oil cooled GSXR engines.

On my stroker I use fully synth because a) it's cleaner burning and b) I like the piece of mind on an engine that spends alot of time being flogged mercilessly. I've never had it nip even after 8 or 9 miles at max. rpm when using fully synth, yet I've had a "phantom" nip using generic 2T oil.
____________________
18:54:48 Rob Fzs: jews don't give away stuff for free

18:54:59 Rob Fzs: unless its their clothes/ hair/ golden teeth

[temeluchus]

[sickpup]

[whitedevil]

[temeluchus]

[Pete.]

Diesel engine oil in my bike changed every thousand miles. Every bike I have used for work has got the same and the only bike I've burnt out a clutch on is my Hayabusa which never had that oil (and was nothing to do with oil) anyway. I use this oil because I get it for free and if I didn't I'd use bike oil but I certainly wouldn't worry if I had to use car oil at all.
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[sickpup]

[temeluchus]

I always assume 1100 for some reason! The 11s are considerably lower strung.
____________________
Some shite cruiser. Now with guns and FREEDOM!

[weasley]

[temeluchus]