I agree that synthetic is slippery and could possibly cause trouble with synchronizers doing their job. I haven't had much experience with synchros because I never use a clutch except for untracking. I even shift 3 on the column without a clutch. But I do know that transmissions shift much easier with 50W synthetic than mineral or GL5. We have logged millions of miles with our Eaton and a few Meritor transmissions with this oil. http://www.finalube.com/Product_Data_Files/Fina_Branded_PDF_Files/Synpro_Transmission_50_pdf.pdf And we actually have less trouble with leaks than we did with mineral oil. Leaks are caused by vibrations and the Synpro eliminates vibrations. I was a distributor of Fina oils for 45 years until the lubricants division was sold to Total. However I still have enough of the Synpro on hand to last me quite a few years.