Car oils should not be used in bikes due to clutch issues as already stated, but also because car oils are not designed to be used in an engine that also houses a gearbox.

The shearing action of the gearbox will break a car oil down and thus it will not provide the correct level of lubrication. There is a difference.

I know this thread is a few months old now but valuable advice anyway.

Don't use fully synthetic... Damnit I just bought myself 4L of BO 10W40 today, haven't recieved it yet though and thus not put it in the bike.

But why exactly shouldn't I use fully synthetic?

I recently discovered the oil called Bardahl. This is an American manufacturer and seems to be one of the most used in classic and racing bikes. It costs more than Castrol but my experiences so far (I have 8 bikes) are great. See http://bardahl.com/about

Use it if you want, mate. As long as it's MA approved, it will be fine, I'd say.

Okey, thanks!

It will be in the bike during the winterstorage anyway, I can change it when spring comes before I start riding it.

If car oil works for you, use it.
I do use bike specific oil but only the cheapest I can get. I still have two 4 ltr. cans in the basement 14 Euros each. There's no need to spend a whole lot of money just for oil. The XT engine is not a high rev engine. So oil is not really an issue imho.
BTW. I use this one http://www.mannol.de/en/?action=acce...=4-Takt%20Plus. It's cheap and it does it's job: lubricating the engine.

No, I do not agree. U must use a specific motor cycle oil as the bike engine is totally different to a car engine. I agree that the XT is indeed a low revving engine but so what, the characteristics are quite special and so the oil is very important and as it only uses about 3L then why save money on oil............

There is no MUST use in motorcycle oil. If car oil works for him and he has never came over a problem with it why not. He is free to use what ever oil he wants.
BTW. what exactly is the total difference between a car engine and a motorcycle engine beside the clutch? Nothing I guess:).

The extreme pressures associated with a gearbox! ;)

I already stated earlier up his page - a car oil is not designed for the shearing action of the gearbox. Fully synthetic is fine, aslong as its bike specific.