wangwilko Posted July 16, 2020 Share Posted July 16, 2020 This may be just a simple question, I have had my differential rebuilt for 36 Desoto and have read a bit about correct diff oils to use. It appears that oils now compared to 80 odd years ago are yards apart. Could someone please guide me or recommend on their experience the correct oil to use and the amount required, I don't want to over fill it and create other issues. Link to comment Share on other sites More sharing options...
keithb7 Posted July 16, 2020 Share Posted July 16, 2020 (edited) Diff oil is not really too concerning. My understanding is modern GL-4 is better for components with brass thrust washers or bushings in them. GL-5 can cause brass to wear quicker? Internet lore.... You can run either in your diff. More concerning is what oil are you running in our transmission? Do not run modern GL-4, GL-5 in your manual, synchromesh transmission. The friction modifiers are too slippery for the ramps that match up adjacent gears speeds with the synchro. You can use a GL-1 available at Napa. Or a Red-Line MTL for example is good for use in your tranny. Edited July 16, 2020 by keithb7 (see edit history) Link to comment Share on other sites More sharing options...
Guest Posted July 16, 2020 Share Posted July 16, 2020 The amount in your differential should bring the level of the oil just up to where you can feel it with the end of your little finger when you feel in through the filler opening. Filling it until it runs out is TOO MUCH. Link to comment Share on other sites More sharing options...
Steve9 Posted July 16, 2020 Share Posted July 16, 2020 In late 1936 Chrysler introduced the hypoid differential. Does that not require a hypoid oil? Link to comment Share on other sites More sharing options...
Bloo Posted July 16, 2020 Share Posted July 16, 2020 (edited) Yes, although most rear axles would be happier with hypoid oil, whether they have hypoid gears or not. Any "normal" gear oil today is hypoid. The exception would be if there is brass inside the rear axle (probably not as late as 1936, but I am not 100% sure about the Mopar axle). If there is brass inside, then you must choose something safe for "yellow metals". That may or may not be hypoid oil. Trouble picking suitable gear oils usually occurs with synchromesh transmissions, not rear axles. Edited July 16, 2020 by Bloo (see edit history) Link to comment Share on other sites More sharing options...
wangwilko Posted July 17, 2020 Author Share Posted July 17, 2020 So trying to get my head around this. The diff oil would be different to the oil used in my engine being thicker? keithb7 mentioned GL-1 which I understand to be gear lubricant and ok for brass? I don't believe my diff has the brass. I am not a mechanic but learning from those with the know how. Off to the shop tomorrow so I want my shopping list to be correct. Link to comment Share on other sites More sharing options...
Bloo Posted July 17, 2020 Share Posted July 17, 2020 (edited) The whole "GL" rating system is a mess. Generally speaking, you want GL-4 or GL-5 in a rear axle. GL-4 and GL-5 are both extreme pressure lubricants. GL-5 is the super duper version. Once upon a time, GL-5 was more expensive, and due to the ingredients needed to make GL-5, it was fairly safe to assume that GL-4 was safer for brass, and also OK for synchromesh transmissions. It's not been true for decades now, and the way the rating system works a GL-5 oil also will pass the tests for a GL-4. There is no difference in cost anymore, so a bottle labeled GL-4 could easily be GL-5. You have to check the datasheet for the particular oil you intend to use for it's "copper strip test" to determine if the oil wants to eat the brass. Thats the long answer. The short answer is if it NOT an old Stutz or something with a brass worm gear, you don't need to worry that much, In a 30s Mopar, the brass, if it exists, would be bushings and thrust washers. I was hoping someone would pipe up who knows whether your 1936 DeSoto has any. If it does, you might want to look at some copper strip tests. Otherwise just put GL-5 in. Edited July 17, 2020 by Bloo (see edit history) Link to comment Share on other sites More sharing options...
Bloo Posted July 17, 2020 Share Posted July 17, 2020 In the "GL" system, if I remember correctly, GL-1 doesn't have a test, it just means "mineral oil". It would be very similar to motor oil, but intended for gears, or a combination of gears and hydraulics, or something. Think of old fashioned 'tractor oil", because that is the intended purpose of any you will find today. The viscosity of gear oils are rated on a different scale that motor oils. 75W80 gear oil sounds thicker than 10W30 motor oil, but it is really about the same. GL-1 does NOT belong in any hypoid rear axle, it simply isn't enough protection. In a non-hypoid rear axle you might get away with it but GL-4 or GL-5 would be much better. Link to comment Share on other sites More sharing options...
jpage Posted July 17, 2020 Share Posted July 17, 2020 I just run a straight 140 wt. gear oil in both the rear end and transmission. Do not use oils that contain high pressure additives that may destroy bronze parts. I think if there is any bronze in the line it might be thrust washers in the transmission. Link to comment Share on other sites More sharing options...
wangwilko Posted July 17, 2020 Author Share Posted July 17, 2020 Thanks everyone so much for your advise and knowledge, all set for my next stage of getting old silver back on the road. 1 Link to comment Share on other sites More sharing options...
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