GL4 gear oil for Muncie SM420 Granny -low

Messages
4,434
Location
Massachusetts
Hey guys, I'm looking to quiet down my old '53 Muncie SM420 transmission in my '92 Wrangler. I currently use Coastal 80W90 (GL3,GL4,&GL5 rated)and it shifts smoothly, but after extended wheeling time, she gets a little noisey and I'm looking for something to 1) quiet it down and 2) add a little more protection. I wheel year round, so I'd like to get at least an 80W-something. I have used Coastal 85W140 (which runs quiet when hot), but it's too thick for winter and I'd like to run this year round. I contacted Amsoil in regard to their "Severe Gear" 75W110, but they didn't think it was a good fit for my synchro's. I'd really like to get this viscosity or even a 75W140 in GL4. Anyone have any ideas? Do you think a synthetic 75 or 80W90 would hold up better to the heat? How about motor oil? I was thinking Syntec 5W50 or Mobil 1 15W50 as most 90wt gear oils are about 14-15cSt when hot. Thanks for your help.
 
Messages
8,756
Location
RI
Yes, there are great fluids 'hiding' at the local truck stops, truck parts places, truck dealerships, and industrial supply houses. Even if you can't find the Mobil1, just about 'ANY' from the Mack list would be a good choice.
 
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