What type of Differential oil?

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dart65

dart65

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Hello I have a 65 dart with a904 trans and 8.25 (8 1/4) suregrip rearend from a 78 aspen.
I only know the rearend is a 8.25 has suregrip and 3.21 gear ratio with large bolt pattern.
  1. What type of fluid should go into this rearend?
    1. I was reading and it seems some say don't use synthetic only use conventional 80w-90 with friction modifier from ford or Mopar
    2. Some say use conventional "hypoid" gear oil 80w90 with friction modifier (BTW what is "hypoid" gear oil?)
    3. Other say synthetic is fine with friction modifier
    4. some say it depends if its a cone or clutch type rearend? (how can I check which it is?)
    5. And still others say 85w-140 conventional gear oil with friction modifier.
Does anyone have a definitive answer? Does anyone have a manual for an old aspen or 70s Mopar that recommends a specific gear oil.
The original 64 Dart manual I have also mentions "hypoid" sae90 lubricant; but this is not a 64 suregrip differential.

Thank you.
 
dart65 said:
conventional or synthetic? or hypoid btw do you know what hypoid is? I cant find it anywhere.
Except this Castrol gear oil mentions hypoid: https://www.classicindustries.com/product/1976/dodge/aspen/parts/cc1840.html

And I forgot some say GL-4 others GL-5. Does the "GL" matter
Click to expand...
I always use 80w90 conventional and add the Ford friction modifier if I have a limited slip unit. These old girls came out well before any fancy synthetics and IMO those are in no way needed on older vehicles. Hypoid refers to the gear type, which includes all automotive ring and pinion gear sets. All automotive 80w90 conventional gear oil is for use with hypoid gears, since all automotive ring and pinions are hypoids.
 
RustyRatRod said:
I always use 80w90 conventional and add the Ford friction modifier if I have a limited slip unit. These old girls came out well before any fancy synthetics and IMO those are in no way needed on older vehicles. Hypoid refers to the gear type, which includes all automotive ring and pinion gear sets. All automotive 80w90 conventional gear oil is for use with hypoid gears, since all automotive ring and pinions are hypoids.
Click to expand...
Thank you.
 
When I got my GTO, it had a slight whine when coasting [ cone type LSD ]. I tried two different oils, no difference. Then I tried Redline 85/140 syn. Got rid of the whine & have used it for 30 yrs.
 
i run this with the ford friction modifier.
https://www.amazon.com/dp/B08KB2XYTP?tag=fabo03-20


.

Screenshot_20250901-090716.png
 
abodyjoe said:
can say the same thing about radial tires but must run radials on these old girls. :)
Click to expand...
I'll say ! Was helping a friend with a 66 Pontiac Catalina the the owner wanted a O.E. restoration.
Took it out for it's first test ride and the bias tires actually scared me. Had to be 50 years ago last time I drove on bias tires.
 
Conventional Valvoline 80/90 gear oil and a 4oz jug of Lubegard or Motorcraft XL3.
 
I just put some royal purple 75-140 with friction modifier in my 3rd this morning. Runs great, but gear oil stinks like crazy.

IMG_2566.jpeg


IMG_2568.jpeg
 

Well, I've done rear axle services on vehicles I know still had the fifty year old plus conventional 80w90 in it and the gear oil looked fine and the gears were all in great shape. If conventional gear oil lasts that long, it's good enough for me. Well over 90% of yall will never put your cars even close to needing synthetic gear oil, so it becomes a total waste of money, but knock it out.
 
You don't need anything special for this diff. Buy Pennzoil if you want to contribute to my pension fund. Otherwise buy cheap and good. Supertech at Walmart is five dollars and change for a quart, and it has the Sure Grip additive already in it. Good stuff, really good value.
If you get any clutch chatter, add a small amount of limited slip additive. Like one ounce of a four ounce bottle. That should do it. Two ounces at most. If you over treat it, the limited slip won't work.
Or get Red Line at $25 a quart. One of the best available, but a poor value for you.
 
TF360 said:
I use the conventional Valvoline that already contains the Limited Slip additive
Click to expand...

Iirc that's the last one I bought. Like 99% of motor oils out there, even the cheapest generic conventional stuff you can get today is way better than the best you could get when these cars were new.

When I was a bit younger and bought into all the synthetic-gear-oil-is-more-efficient rhetoric I changed out the gear oil in the front and rear diffs on my 1993 Jeep Cherokee daily driver (which also had an 8.25" rear end btw) to synthetic hoping to gain some MPG. Did I? Maybe but not enough to counteract with better driving habits, meaning 0.5 MPG at best. Also not enough to offset the initial cost of buying all that gear oil in the first place. Live and learn...
 
S'cuder said:
You don't need anything special for this diff. Buy Pennzoil if you want to contribute to my pension fund. Otherwise buy cheap and good. Supertech at Walmart is five dollars and change for a quart, and it has the Sure Grip additive already in it. Good stuff, really good value.
If you get any clutch chatter, add a small amount of limited slip additive. Like one ounce of a four ounce bottle. That should do it. Two ounces at most. If you over treat it, the limited slip won't work.
Or get Red Line at $25 a quart. One of the best available, but a poor value for you.
Click to expand...
All of the Super Tech oil products routinely score high in independent testing.
 
Thank you for all the feedback, I will start with these -Castrol 80w-90 and Motocraft -XL3. I may experiment with Redline if there is any noise.
1756765017998.png
1756765057921.png



Does anyone know of a thread with 8.25 differential DIY maintenance? I found the following link on this forum but am unsure if this is for vintage differentials or new Jeep 8.25".

It seems fairly straight forward and please correct me if I am wrong:
  1. Jack the car up on jack stands braced against the rear frame letting the rear wheels hang down and use front wheel chocks
  2. Remove the 10 bolts out of the differential cover
  3. Remove the differential cover and let the oil drain
  4. Ensure the differential case hole is facing down if not the engine can be used to turn the diff until it is facing down and drains
  5. clean the cover mating surfaces with acetone or mineral spirits remove all old gasket.
  6. ????use silicon gasket maker of Fel-Pro cardboard gasket with silicone?????
    1. which is better a carboard gasket or simply Permatex gasket maker??
    2. Or am I supposed to use carboard gasket with Permatex high-tack
  7. reapply the differential cover
  8. Lowe the vehicle to level ground
  9. take the filler plug out
  10. Fill with 2.2 quarts of gear oil +friction modifier
  11. replaced the fill plug and drive away
 
dart65 said:
Thank you for all the feedback, I will start with these -Castrol 80w-90 and Motocraft -XL3. I may experiment with Redline if there is any noise.
View attachment 1716449332View attachment 1716449333


Does anyone know of a thread with 8.25 differential DIY maintenance? I found the following link on this forum but am unsure if this is for vintage differentials or new Jeep 8.25".
[/URL]

It seems fairly straight forward and please correct me if I am wrong:
  1. Jack the car up on jack stands braced against the rear frame letting the rear wheels hang down and use front wheel chocks
  2. Remove the 10 bolts out of the differential cover
  3. Remove the differential cover and let the oil drain
  4. Ensure the differential case hole is facing down if not the engine can be used to turn the diff until it is facing down and drains
  5. clean the cover mating surfaces with acetone or mineral spirits remove all old gasket.
  6. ????use silicon gasket maker of Fel-Pro cardboard gasket with silicone?????
    1. which is better a carboard gasket or simply Permatex gasket maker??
    2. Or am I supposed to use carboard gasket with Permatex high-tack
  7. reapply the differential cover
  8. Lowe the vehicle to level ground
  9. take the filler plug out
  10. Fill with 2.2 quarts of gear oil +friction modifier
  11. replaced the fill plug and drive away
Click to expand...
That will work well. Since that is limited slip gear oil with additive already in it, hold off on putting in additional additive until you hear clutch chatter. Which you shouldn't.
 
S'cuder said:
That will work well. Since that is limited slip gear oil with additive already in it, hold off on putting in additional additive until you hear clutch chatter. Which you shouldn't.
Click to expand...
Thank you I missed your first post about 1-2ounces of friction modifier. I take it my DIY steps are correct? would you recommend a cardboard gasket on the diff case cover?
 
Your steps are good. Let me give you a hint or two. Loosen, but leave a couple bolts threaded on the top of the cover. Then gently pry the bottom of the cover open, letting the oil drain slowly before removing it. If you drop it all at once, you could have a big mess.

I like using just gasket maker, but I've used gaskets both gooped and dry. It all works. If your cover is dinged up at all, you may want to use a gasket and a small amount of RTV. I like to let gasket maker cure for 24 hours before adding gear oil.

If you hear chatter, add 1oz. of friction modifier. Then go to a parking lot and drive some figure eights. The additive will get well mixed, and the tight turns will cause chattering if you need to add still more.

Good luck, you got this.
 
S'cuder said:
Your steps are good. Let me give you a hint or two. Loosen, but leave a couple bolts threaded on the top of the cover. Then gently pry the bottom of the cover open, letting the oil drain slowly before removing it. If you drop it all at once, you could have a big mess.

I like using just gasket maker, but I've used gaskets both gooped and dry. It all works. If your cover is dinged up at all, you may want to use a gasket and a small amount of RTV. I like to let gasket maker cure for 24 hours before adding gear oil.

If you hear chatter, add 1oz. of friction modifier. Then go to a parking lot and drive some figure eights. The additive will get well mixed, and the tight turns will cause chattering if you need to add still more.

Good luck, you got this.
Click to expand...
Thank you again.
 
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