can i mix oils? and also why the heck is there no 5w40 mobil one sythetic??
 

my moms car is low on oil about 1 litre, there is 5w40 mobil one sythetic in there.. can i put in 5w30 or is mixing oil no good? the reason why i ask if i can mix oil is cause well there is no damn 5w40 mobil one sythetic WHY!!!

I dont see why not. But where are you located that you cannot get 5w40??? Its everywhere!
Lordco/Mopac etc etc.... aslong as the 5w30 is synthetic aswell and made by Mobil 1 and is an approved MB oil.

Yeah, if it's just one litre, you can (within reason) put just about any motor oil in there without damaging anything.

I wouldnt mix synthetic with the conventional oil....mixing that could cause a whole veriety of problems...foaming; less lubrication and they won't mix together...

IRRC, he drives an MB, MB cars depends on clean full synthetic oils...mixing oil weight (within reason) will not affect it short term but mixing types of oil will.

It depends on the oil you use, read the label, they will say whether they are compatible or not. RTFM. There is mobil1 0W40.. I use that one for my BMWs. You can now get it at Canadian Tire (at least the big ones eg on Grandview) even though at a higher price than at wholesale.