Originally Posted by BusyBenz

Recently I changed the tranny fluid in my girlfriends S70 Volvo. It had 61K on it, and the old tranny fluid was nearly black, or very dark brown. I sold here on the idea of synthetic Mobil One tranny fluid. The tranny system in her car is 2 gallons. I was able only to drain 1 gallon (no drain for torque converter) and I didn't want to do the trick of removal of one of the tranny cooling lines and run it until nothing came out, so I only put back in the 1 gallon I drained using the Mobil One.

Will a 50% mix of M-1 fluid, and 50% mix of old dyno fluid present a problem?

I plan to change fluid again in 15K, thus diluting the old with new even more.