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I've put 20,000 miles on my truck since I bobbed it and I've had 30w in the transfer case for most of those miles... and it hasn't exploded! I haven't switched over the transmission, for very technical reasons, the fluid is clean and I'm too dang lazy and cheap! I had to rebuild my transfer...

Stuart, I double checked what i thought I knew, and you are right! Attached is a viscosity chart. Smokewagon66, I've got the manufactures documentation regarding the transfer case (it's posted here in the thread) and their recommendation for SAE50, but don't have any docs for the...

85W90 is way too thick. SAE50, good!

Specific to the transfer case; The manufacture's recommendation of GL-1 or SAE 30, 40 or 50 is the best choice. A yellow metal safe GL-5 probably would be ok.... as long as it's a air-shift. My transfer case is a sprag unit, and works perfectly (just put in fresh sprags). I think GL-5 would...

Rockwell, the manufacturer of your T136-21 transfer case states 80w or 90w GL-1 or.... 30w, 40w or 50w CD spec oil (diesel motor oil). Attached is the page from the Rockwell manual.

There is another thread going regarding transmission oil, and it points out that the current Lubrication Order doesn't specify GL-1 it says use MIL-L-2105 80w/90. Castrol EPX gear oils are MIL-L-2105 spec, I'm sure many others are too. So even the military is discontinuing GL-1. Mike W

When I started this post the point I was trying to make was that, while GL-1 spec gear oil is a fine lubricant, it was chosen because of availability in the supply line and a good choice at that time. Move forward a few decades and there are newer spec oils with improved properties...

That's the thing, the transfer case has yellow metal syncros too. Meritor specifically warns not to use a GL-5 gear lube because it will eat yellow metal. Motor oil doesn't have the sulfur based additives that attack copper alloys but it does have lots of corrosion inhibitors. Many manual...

I don't like GL-1 gear oil and wanted something "better" in my transfer case and transmission. GL-1 is hard to find, and finding one with a corrosion inhibitor is even harder! So I called my local HD truck driveline shop, Blumenthal's. They gave me the tech support phone number to Meritor...