most motorbikes have a 'wet' clutch (as we say in german). means the clutch is penetrated by engine oil. additionaly, in most motorbikes the engine oil has to grease the gears as well. and finally, usually motorbike engines revolve faster than car engines do.
so the oil inside a motorbike has to fullfil more and higher demands than it has to do in a car engine.
nevertheless, as long as you ride your xt 'normal' under normal middle-european conditions, car oil will do fine (racing or crossing the sahara with full throttle and lots of luggage is something different of course).
more important is the api-code. yamaha wants at least SE-classification for the xt. which in fact is the lowest quality you can buy nowadays.
so don't worry very much about oil quality, just ride her gently and let her have a regular oil change with whatever car or bike oil you may find around the globe.