My Audi calls for GL-4 75W oil for the manual transmission. I'd like to use a mix of Red Line MT-90 and MTL, both GL-4 lubes. The MT-90 is 75W-90 and the MTL is 75W-80. Most Audi racers and those in cold climates use MTL and the non-racers and warm weather people use MT-90. Since I do a little racing every now and then, and I live in Northeastern US with greatly varying temperatures, I want to use a blend of both all year round. I assume since this is the same manufacturer and type of oil, it should be Ok and I should end up with a blend somewhere around 75W-85? This should be safe to do, right?

Congratulations on selecting Redline for your tranny. I have a manual tranny that spec's ATF and am running Redline D4 ATF in it.

I doubt you would have any problems if you mix the MTL and MT-90, but I would not do it except you first call Redline and talk to their tech person. I believe his name is Dave and he is quite willing to discuss your applications.

BTW, not sure where you get your Redline, but if mail order myoilshop.com in Pennsylvania has the best price I have seen--most Redline is $7 per quart there. I believe they also have some tech folks and apparently exclusively deal in Redline oils. May run your question by them too.