How much oil does an xr100r take?

one quart 10w40 four stroke motorcycle oil. do not use car or truck oil.

10w40 4 stroke oil is car & truck oil which are also 4 stroke engines. You can use the very same oil you use in your car and truck as long as you use the correct weight which is 10w30 and not 10w40. There is no difference in so-called motorcycle oil and car or truck oil. I do however highly recommend you use full synthetic oil in any air cooled engine.