Is it safe to mix oil viscosities from the same brand and same weight?
For example, if I had a 50/50 combination of Mobil1 5w30 and 10w30, would it be safe to do so and would the resulting mixture be a 7.5w30??

How about different weights between the same brand? 5w30 and 15w40?

 

last i heard it wasn't worth the trouble

 

Even if it isn't even worth the trouble, would it be safe?

 

Mixing weights to form a witches brew isn't worth the trouble. The weights don't average out like you'd think and even on the same brand, the different viscosities have different formulations. The different formulations might not be too compatible. On a short interval, this won't be a problem. On a extended one, it could be.