If i was to take 2.5 qts of 10w-30 and mix it with 2.5 qts of 1550 would i get a 12.5w-40?

 

probably not, but you wont hurt anything buy mixing 2 different viscosity oils. most oil additives are just an extreamly thick type of oil..at least that what my auto instructor in college told me..he has his phd in physics and has been building race engine for over 30 years so, i trust him..

 

You won't get an exact mix, because each of those two oils uses a different base oil and different levels of viscosity index improvers in order to achieve their viscosity. So mixing is never just an average of the two. If there is a specific viscosity you want you're better off buying the oil that way to begin with.