Differential fluid

[1969CoronetR/T]

I wanted to change the fluids in the differential in my 1969 Coronet R/T and was told not to use synthetic. I have a case of Royal Purple 75w-140 synthetic, but my friend claims it will ruin the seals and just use regular (non-synthetic) gear oil. Is he correct?
Thanks,
Mark

 

[RustyRatRod]

It will not "ruin" the seals. However, synthetic has such great flow characteristics that it can find leaks in seals where a carbon based oil may not. I am sure that's what your friend is referring to, and he's not wrong. I'm not saying synthetics are bad, but you may find a seeping seal. If the seals have any weak spots, the synthetic will find it.....maybe that's the best way to put it.

 

[1969CoronetR/T]

How many seals are in the 8-3/4" ? I am weighing if I should use the synthetic and take my chances with the old seals, use the synthetic and change the seals or just use non-synthetic gear lube.
Thanks,
Mark

 

[Grabinov911]

Hey Mark for what it's worth I have a new 3.55 True Trac rear end that I bought from Dr. Diff and he said the same thing - Dinosaur oil only. I didn't get into the "why" of it, but I did put in new seals, which I got from him also, and he still said no synthetics...

 

[Photon440]

I'm not impressed with people who cling to the past like that. I'm a fan of Amsoil synthetics, including their severe gear 75-110 gear oil. I'm of the opinion that lubrication and oil chemistry has helped synthetics advance past what the old dino oils could deliver. If it takes a new seal to take advantage of lower temperatures, reduced friction and longer life, that's fine with me.

And like I said...it's my opinion. So far synthetics haven't steered me wrong.

 

[Dennis H]

I'm not impressed with people who cling to the past like that. I'm a fan of Amsoil synthetics, including their severe gear 75-110 gear oil. I'm of the opinion that lubrication and oil chemistry has helped synthetics advance past what the old dino oils could deliver. If it takes a new seal to take advantage of lower temperatures, reduced friction and longer life, that's fine with me.

And like I said...it's my opinion. So far synthetics haven't steered me wrong.

Same here. I use the Amsoil severe gear in the Dana. Synthetic in the motor and TKO trans as well.

 

[eric383]

X2 Rusty. The only issue with synthetics are the flow characteristics. On a used rear diff. you may have some weeping past the seals. I have personally experienced this on a daily driver an woundup replacing axle seals. Just my 2 cents worth.

 

[1969CoronetR/T]

It has steep gears, I think 4.10:1 and would like to use the synthetic that I already bought last year for the differential. I never got around to it and now it needs changed. Thanks for all the help.
Mark

 

[kb67mopar]

The only thing i can say is make sure you always use the recommended viscosity. As an engineer I have been around OEM testing and can tell you they design the bearings, seals, etc all for a specific weight, type and detergent package. So I always recommend something that matches OEM standards synthetic or not. Though when given an option I choose synthetics.

 

[67 coronet]

use the synthetic.if it leaks,you prob needed to change the seal soon anyhow.synthetics are much more stable then the dead dino oils.

 

[1969CoronetR/T]

I used Royal Purple 75w-90 gear oil that is fully synthetic. I did a few circle eights and drove it to the beer distributor. Will have to wait to see if it leaks throughout the night. Luckily, I remembered to clean the garage floor after I washed the R/T.