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[BazaJT]

I run the Elite using 10w40 oil,by chance my 125 bike also uses 10w40 oil.However according to the manual I can't use the same oil for both engines.It states in the bikes manual that you must not use an oil meant for use in a car engine,why?What does one have that the other doesn't?

[Lazydocker]

The theory is about the "Wet Clutch" fitted to many bikes.

But I believe TB used to run his scooter on GM's finest until he melted it

[Migv6 le Frog Fan]

Some car engine oils contain friction modifiers which will wreck your clutch plates , assuming your bike has a wet clutch.
GM 10/40 semi doesn't contain these though, so if that's what your using in your car it will be fine in your bike. Other brands you would need to check.
I used GM 10/40 in bike engines for years (often at very high revs) and had no ill effects. 

[BazaJT]

The oil I use for the car is Triple QX from Eurocarparts.It's not that I mind buying a separate oil for the bike,just wondered why it said I couldn't in the manual.

[baggers]

This is a discussion that has gone on for years, some people claim they don't have any problems others to the contrary.
Wet clutches are going to suffer with the wrong oil, that is something that needs consideration. 

The oil in the car is designed to lubricate, manufacturers have designed it knowing what it is going to be subjected to but put that oil in a bike and it's going to see 18K revs instead of 6K and it's lubricating a gearbox which would need an element of EP properties and that must be 'just for starters'.

It would be interesting if oil price was like-for-like would you put bike oil in your car? 

[Mr Gav]

I think it also has something to do with the engine oil also lubricates the gearbox in bike engines so oils for bikes are made with this in mind, after all you dont`t go putting 10/40 in your R28 do you 

[BazaJT]

What's an R28 when it's at home Although I will be buying the correct oil for the bike I don't particularly see that running a gearbox with engine oil should be a factor after all minis,1100/1300's do it.

[baggers]

They did and they were on 20 50 Castrol GTX liquid engineering,  Although mine was running R40  (lovely smell).

[Migv6 le Frog Fan]

Expensive motorcycle oil is just designer snake oil. No need for it at all. The reason there is so much discussion / argument, is that most people don't bother to find out the facts.
I did, and binned the £12 per litre stuff in the fancy container and used good old GM 10/40 semi.
Ive used it for several years and often rev the engine to 13k rpm. The engine has had no ill effects whatsoever.
And your right. Gearboxes have nothing to do with it.
Up to you though, its your money. 

[dbug]

They did and they were on 20 50 Castrol GTX liquid engineering,  Although mine was running R40  (lovely smell).

Ah Castrol R - brings back memories of my racing 1293 S, all those years ago   I can smell it now!!

[BazaJT]

Hmmm,seems to be a bit of disagreement going off on this.Basically although it's a 12 plate bike it was/still is a bit of a resto project with the intention of using for my commute to work.Bike is red lined at 10,000 but I can't see me giving it that much welly,but don't want it breaking for want of the proper oil.

[simON73]

The issue is with clutches. If you use car oil for your bike you can mess-up your clutch as the friction modifiers in most car engine oils use special additives to make the oil even better in lubricating. The additives to oil for cars mean less friction.
 
So my recommendation is to stick with special bike oil

[TheBoy]

I ran GM 10W40 semi in the ZX-10 for years, with no issues relating to the clutch. And I enjoyed using all arse-clenching 12k of the rev range quite frequently.  The same oil also went in my previous Kwaka, which was also a wet multiplate clutch.

If I still had it, it would still be on a annual/4k diet of GM's semi.


As others have said, some oils have friction modifiers in that are incompatible with wet clutches. GM's semi cause no issues, hence it went in everything with an internal combustion engine here, including the mowers.