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Been having issues keeping her in first and found this on the jeepsunlimited.com (thanks to user marc1kim), thought it'd be useful to share. This info is only for the Aisin (AX5/15 5 speed manual) transmissions, not the BA-10 or AW4: *Pennzoil Syncromesh, and Royal Purple Synchromax are only recommended for the NV3550 and other NV trannys. (Not recommended for AX-5/AX-15). *Pennzoil has a GL4 lube, and Royal Purple recommends 10W40 motor oil for all 98 and newer jeep manual transmissions WITHOUT the NV trans. (Ax-5/Ax-15 fall in this category) *10W30 and (Jeeps recommended) GL-3 are roughly the same viscosity, and Gl-4 and 10W40 same viscosity as well. *All GL lubes have some kind of Sulfur content in them (needed for the High pressure Hypoid gears in rear ends, but not necessarily needed for most transmissions.) *Sulfur addadives react to brass internal parts. *GL-5 has twice the sulfur content of GL-4. *AX-5 and AX-15 are japanese manual transmissions, and the majority of manual Japanese transmission do not recommend the use of GL-5. because of the way it reacts to brass. *Red Line MTL is classified as a 70W80 gear lubricant satisfying the API Service requirements of GL-3 and GL-4. MTL also satisfies motor oil viscosities SAE 5W/10W/30, and the viscosity requirements for ATFs. *Redline MT-90 is a 75W90 GL-4 and also satisfies motor oil viscosities SAE 10W/15W/40. So it's safe to say: AX-5/AX-15 can use (synthetic?) 10W30, 10W40, GL-4, Or Redlines MT-90 or MTL and the New Venture transmissions Pennzoil Synchromesh, and Royal Purple Synchromax. Allthough I searched many website Here are some references with some good info: Good basic info and a mention of GL/ motor oil equivalents, and one mention of Japanese gear boxes. http://en.wikipedia.org/wiki/Gear_oil Viscosity chart: http://www.bobistheoilguy.com/visc.html Gear Lube Definitions: http://www.finalube.com/reference_ma...tegories.h tm Redline MTL / MT-90 info and what they can substitute: http://www.roadraceengineering.com/p...-mtl-info.html And from the Amsoil website: "The difference in GL-4 and GL-5 is that GL-4 gear lubes have half the extreme pressure additives of GL-5. Because the gear types in manual transmissions do not necessitate the use of GL-5 gear lube, GL-4 is the correct recommendation called for by most OEM’s when gear lube is required." GL-4 definition: "This designation denotes lubricants intended for axles with spiral bevel gears operating under moderate to severe conditions of speed and load or axles with hypoid gears operating under moderate speeds and loads." - so GL-4 will have additives for the Hypoid Gears. And here's where it all comes home (and probably explains why Chrysler made the change): GL-3 Definition: "Gear lubricants designated for API GL-3 service are not intended for axles with hypoid gears. Some transmission and axle manufacturers specify API Category "CC" or "CD" motor oils for this service." note: "CC" and "CD" oils have been upgraded to newer designations and are equivalent to 15w40

Hey everybody, I've been trying to change the fluid in my '89 4.0 5-speed for about a week now. Started off getting interrupted by surprise storms, then realized I didn't have a 15/16" wrench so I ordered one, then realized I've got a seriously stuck fill plug and it looks like the previous owner lent it to a teething baby 'cause it's all kinds of chewed up. Couldn't get it off with the 15/16" and started doing some research. Been soaking it in a little WD-40 but I'm about to go pick up some PB Blaster. Saw a few mentions of trying a pipe wrench, but that didn't work. Also saw mention of hitting it with a torch but haven't tried that yet. So my question, and I'm hoping it's a very stupid and unnecessary one but it probably can't hurt to ask... Righty tighty, lefty loosey, correct? Been trying to remove the plug in the direction I'd expect it to loosen, but I'm going to feel like a real idiot if this thing is threaded the other way for some reason. Also should mention that I haven't drained the fluid so I'm not in a position where I've had to pull the shifter and fill it that way, and the truck shifts fine, but I bought it last month and I have no idea when it was last changed so I'd sure like to get it done. Thanks in advance!