I'm looking at buying a rebuilt SM420 for my '51, and I've been doing some research on gear oil. From what I can glean from the internet, GL-5 oil is out. I have read some good reviews of Sta-Lube 85W90 GL-4, but it seems a bit pricey at almost $38 a gallon. Then I found that Coastal has 90 weight in both GL-4 and
GL-1, at about $10 a gallon . Does anyone have any experience with the Coastal oil? Any opinions on GL-1 versus GL-4? GL-1 seems like it would be more in line with what was available 50 years ago, but that may not be the best.
Hopefully, things here don't get as heated as they do over motor oil preference.
