In my opinion the brand matters very much. I have spent 30yrs trying to disprove the claims from Amsoil and failed each and every time. Does it really matter, I dunno, but I have had nothing but stellar service from all Amsoil products I have used which is a lot of them.
I have done yearly oil changes and every 2-3yrs on differentials, transmission and transfer case on every vehicle we own for decades. The 2003 Monte Carlo has 200k miles and 2004 Expedition has 285k miles both bought new. The 2000 Ranger was bought used with 130K miles now has ~180k miles and the 2002 Monte Carlo was bought used with 5,900 miles(it was a garage ornament). I drained/flushed the transmissions the first time best I could and now I drop the pan and change the filter every 2-3yrs depending on use.
Anyway, to the best of my knowledge there remain just 3 street use 100% synthetic oils. Amsoil, Royal Purple and Red Line. Amsoil shows their test results whether they were the best or not, that means a lot to me. Fully synthetic does not mean 100% synthetic, it means 52% or more.
My 1994 Ranger blew through front wheel bearings every 5-10k miles, I put 124k miles on it in 18 months. I had not switched to Amsoil grease at that time. Once I tried Amsoil grease I never had another front bearing failure, ~75k miles and then I sold it. The truck had 6" of lift and 35" tires was used for construction and towing during the week and 4 wheelin' on the weekends.
My 1979 Bronco would not stay running in the winter on cold startup without constantly rapping the throttle for like 5 minutes, switched to Amsoil engine oil and it would fire up and stay running with no more than 1 tap on the throttle.
I have no interest in a debate on lubricants, this is just my experience since I switched to Amsoil in 1991. Even if I'm wrong, it makes me feel better to use it and that is worth the price to me.
