Ford recommends 75W-140 synthetic (never need replacing). Since I'm changing trans fluid (60,000 miles for 5-speeds) thought I might as well do rear. Motorcraft web site does not list a 75W-140 synthetic (only 75W-140 high performance and 75W-90 synthetic also a 75W-90 fuel efficient high performance). Recommended fluid for F150 is also synthetic 75W-140 and this needs to be changed if your towing. So what gives? Mobil makes a 75W-140 synthetic (at least their web site lists one), but local stores don't have, only 75W-90. On place had Coastal semi synthetic75W-140 and Valvoline full synthetic 75W-140 (but only one quart and I need 1.5 quarts). I know Amsoil makes one but I'm not a fan (if your one of their distributors don't post telling me how it's better than Mobil, I already know that). Not too worried but what should be used?