Here is what RP has to say about the various 10W-40 oils they make as well as why Synchromax is no longer on this chart of theirs. I am not satisfied with the response, as I wanted to know why the change with the 2012 - 2013 RP Chart. RP has had 10W-40 oils for some time; what caused the switch in their recommendation was the specific question I asked of RP.
"I have only been in the tech department here at Royal Purple since May of 2011, however unless it was a race application the MTX-75 transaxle recommendation has always been a 10W-40 performance oil. I’m not sure if that was changed from before because I was not here, though my supervisor Chris has been here 2 ½ years longer than me and he indicated it has been that way for a good while.
The 10W-40 recommendation is what best matches the original Ford fluid viscosity, this matters because shift feel, smoothness and quietness are best optimized this way. There is no question Synchromax works and has more than enough protection, but it may not feel correct as it is closer to a light 20 weight on the engine scale. We used to recommend our old formula SAE API-SL street oil 10W-40, but that is now an API-SN and as a result of the SN chemistry restrictions it now has less anti wear additive than we feel is appropriate for gear box usage.
Any of our performance 10W-40 oils will work great in the Ford MTX-75.
Royal Purple HPS 10W-40 (part #31140, quart bottle)
Royal Purple Max Cycle 10W-40 (part #01315, quart bottle)
Royal Purple XPR 10W-40 (part #01041, quart bottle)
Thank you for choosing Royal Purple and have a great day!"
Jim Morrissey
Technical Services/Automotive
Royal Purple, Inc.