i put in royal purple in my front diff and i came up short maybe 1/4 of a quart. can i throw in some same synthethic oil but not royal purple?

 

yes, you could top off with anything.

 

oh yea. it doesn't evn have to e synthetic?

 

No, it should still be synthetic. I would try to get more RP and top it of with that. Different oil companies use different methods and mixtures in their oils, I always use the same oil in everything, and I try to never mix oils.