Would it be okay to mix Redline Gear oil 75/90 with MT-90 75/90 for my differential?  I have half quarts of each left from previous changes and I want to change out the oil in a R180.

Thanks for any replies, I haven't been able to find a definate answer searching the internet.

Should be fine.  If I recall right, Redline even says on the label that it's okay to blend it with plain old "normal" diff oil.  It is in their FAQs for sure.  At the bottom.

https://www.redlineoil.com/techinfo_faq.aspx

Thank you Sir! 

Of all the googling I did I never thought of going to Redline's website.