Doug, this will conflict with Evan's post, but here goes. 90W gear oil is what was supposed to be used in the SM420's. It is hard to find, so some people use a multi-grade such as 80W-90. I have heard, but cannot verify, that the multi-grades have ingredients that are detrimental to any brass parts that are in the tranny. I stick to straight 90W in both my tranny and my Eaton HO52 rear end and have no issues.
As hard as it may seem to find, it is under all of our noses at Walmart. Go to the marine section in sporting goods. Look for Quicksilver (Mercury) High Performance 90W lower unit gear oil. It isn't cheap at about $10 per quart, but you don't need much. They also sell a Quicksilver Premium multi-grade 80W-90 gear oil...don't buy that. Here is a link--
https://www.walmart.com/ip/Quicksilver-High-Performance-Gear-Lube-1-qt/16565456I have also seen posts on this forum that Tractor Supply sells 90W mineral oil. I have never tried it, but it is much less expensive.
https://www.tractorsupply.com/tsc/p...90-transmission-fluid-2-gal?cm_vc=-10005Matt
