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Well you seem stuck on using it, so have at'r ! 
What weight oil should I use in my 67 Coronet's 8 3/4 axle?
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It's already in, he's just trying to feel better about his decision.Well you seem stuck on using it, so have at'r !
On an original differential I really think conventional none synthetic is a must. Same thought process for gear oil in our old four speed transmissions. They require a brass friendly GL4 only. Not a GL4 GL5 compatible. I had to special order straight GL4 just recently, not on the shelves.
Honestly, I'm not trying to be difficult and I'm sorry for getting into this thread. No, I'm not trying to feel better about my choice. I'm trying to find some specific reasons why the synthetic should not be used in an old open diff. So, from above, what is brass in my old diff to justify comparison with a manual transmission? The discussion needn't go on if you all are tiring of my questions.
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I'm not an expert. My friend was. I gave his reasons for not using synthetic in post #10.
Maybe a better question is,
What are you hoping to gain by deviating from what works perfectly well?
What are you hoping to gain by deviating from what works perfectly well?
dodgemahal
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I wish I could remember what he said. Dr Richard Ehrenberg stated in Mopar Action tech articles recently that the wrong viscosity can cause problems. Nothing catastrophic, but problems none the less. Like noise maybe? Just sayin.
Some of the same type of gains that are realized by using synthetic oils in engines and transmissions for which there is a fair amount of published research.
Certainly, wrong viscosity is a bad thing. I'm not sure how that is relevant here. Today's dino or synthetic 75W-90 is perfectly correct to use for yesterday's single weight 90 - better if you have a real cold start.
Yes, you did - I'm sorry I didn't look carefully enough..."synthetic didn't cling enough, lube the bearings well, or provide enough pressure cushion. I'm sure it was a personal preference, derived from LOTS of experience".
Those are fair comments but published research from various makers like Mobil, AMSOIL, etc. at least seem to question those concerns.
I do thank you for providing good information for me to chew on and I appreciate you (all) taking your time.
Just had a nice convesation with a very knowledgeable guy from Moser at the PRI show. He said not to break in gears with synthetic. Afterwards no proble. He sais synthetic lubes bearings very well. The gear itself during break in not so much.
Doug
Doug
john-9
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i have been using synthetic oils in my differentials , engines , transmissions since the early 90's in every vehicle i've owned and still own ( 1977 F150 , 1972 Mustang ,) and the wife's Liberty , Grand Cherokee , Dodge Caravans etc. she's owned including lawn mowers and snow blowers and have had zero problems -- i have a problem with the previous post statement " not to break in gears with synthetic oil " so could someone please explain to me why ALL the auto companies have been using synthetic oil in their differentials since the late 90's if not earlier . look under that Ram , F150 , GMC , and rear drive cars such as Mustangs etc. and the tag on the diff will say use synthetic oil only usually in the 75-140 weight , i am sure they did not break in the diffs with dino oil then changed it out to synthetic before shipping the vehicle .
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The current discussion is not about 90’s differentials or any vehicles you are referencing. And Fords… Yuck!
BSB67
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I used synthetic in my 8 3/4 sure grip 25 plus years ago after new clutches were installed. Added stinky stuff a little bit at a time. The SG started working properly at about a 1/2 bottle. Lots of street miles and track passes.
66Satellite47
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With a low impact use vehicle I doubt it makes much difference. 75-90 synthetic should be fine. With a limited slip, especially the clutch type, the friction modifier is really required.
This does speak to my particular situation, an old original set of gears, not a new set to break in and in normal use without any hard acceleration or high heat. Thanks for the input.
I really thank you all for the good discussion here. I learn a lot of details about Mopars here that are helping me get back into mechanic work on my old Dodge after not doing much in recent years with newer vehicles. Next job, refresh the braking system. Wish me luck!
dodgemahal
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Eureka! I found it! Here's what Dr. Ehrenberg, SAE had to say in an article about maximizing mpg in our older mopes:
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