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.