Which weight gear oil do I need???

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FF is 80w90 dino except for towing pkg. Mine has towing pkg and the manual specifies 75w140 syn was FF for that. I have always used 75w140 in the rear diff accordingly, but now I am abandoning synthetic gear oils and I need some advice. I only tow a light trailer usually (under 2000lbs). I am worried about cold-weather flow of the 85w140 dino and I am thinking of just using 80w90 dino in the rear diff, despite what the manual says. What do you think I should do? Now summer is approaching but I do not want to have to do a diff service for each season. I ran the thicker stuff in the synthetic because I was less worried about its flow characteristics and I suspect that is why the OEM recommended full-syn for the thicker gear oil in tow pkg applications. This is on a '98 Grand Cherokee with the Mopar Trac-lok limited slip & a dana44a axle.
 
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I think you should use synthetic 75W90 even it costs a little more. The reason is you need to change diff fluid every 3-4 years. The extra cost every year is no more than $2-3.
 

FastSUV

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 Originally Posted By: Zaedock
I run 80W90 Supertech or Coastal in all my Jeeps' axles from trail rigs to drivers. It will be fine.
Well I know Coastal is cheap and I am using a similar oil in the front diff (3 bucks a qt), because the front is an open diff. But for the rear, I have to get some stuff that has the limited slip additive already in it, so I always use name-brand oil in the rear.
 Originally Posted By: HTSS_TR
I think you should use synthetic 75W90 even it costs a little more. The reason is you need to change diff fluid every 3-4 years. The extra cost every year is no more than $2-3.
As mentioned, I have been using full-syn oil already and I am going to stop. This is a weekend warrior and not a daily driver so the gear oil will have to be changed more frequestly due to water contamination, etc. Otherwise, yes you are correct that synthetic will last longer.
 
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 Originally Posted By: FastSUV
This is a weekend warrior and not a daily driver so the gear oil will have to be changed more frequestly due to water contamination, etc.
Just don't push that D44a too far. The center section isn't known for longevity. When it time comes, shop around for an XJ Dana44 and get some mounts from Currie. A Ford 8.8 would be a good option for you too.
 

FastSUV

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 Originally Posted By: Zaedock
 Originally Posted By: FastSUV
This is a weekend warrior and not a daily driver so the gear oil will have to be changed more frequestly due to water contamination, etc.
Just don't push that D44a too far. The center section isn't known for longevity. When it time comes, shop around for an XJ Dana44 and get some mounts from Currie. A Ford 8.8 would be a good option for you too.
Yeah I am aware of that...I have on my "list" to truss the 44a or get an 8.8, but I have done axles, carrier bearings, outer bearings, and rebuilt the posi before...I know this axle too well LOL...but I am just wondering if I can run the 80w90 Mystik with no issues instead of paying for synthetic all the time since I seem to drain it quite frequently?
 
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 Originally Posted By: FastSUV
...I know this axle too well LOL...but I am just wondering if I can run the 80w90 Mystik with no issues instead of paying for synthetic all the time since I seem to drain it quite frequently?
I know what you mean when it comes to the parts in your rig. \:\! FWIW, I ran a Dana35 with a Lockright and Yukon alloy shafts for MANY years on 35's running tough trails, extended highway, even been up Mt Washington (NH) a half dozen times. I always ran SuperTech 80W90. I think your Mystik will be perfectly fine!
 

FastSUV

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 Originally Posted By: Zaedock
 Originally Posted By: FastSUV
...I know this axle too well LOL...but I am just wondering if I can run the 80w90 Mystik with no issues instead of paying for synthetic all the time since I seem to drain it quite frequently?
I know what you mean when it comes to the parts in your rig. \:\! FWIW, I ran a Dana35 with a Lockright and Yukon alloy shafts for MANY years on 35's running tough trails, extended highway, even been up Mt Washington (NH) a half dozen times. I always ran SuperTech 80W90. I think your Mystik will be perfectly fine!
That makes me feel better...I am getting slammed on the Jeep board for daring to run anything other than what my owner's manual says...well I ran 75w140 for years in the rear of the ZJ. I even liked the fact that I did not have to add the Mopar Friction Modifier because the RP had enough in it already for the posi to work correctly. But RP is garbage from recent studies I have read and even if it was the best oil since sliced bread, I hate to drain 50 bucks worth of gear oil out everytime I need to get into the pumpkin. But the Jeep folks insist that since mine is tow pkg and had the trac-lok I MUST run 75w140 synthetic...I have always done this, but I want to have a case of gear oil to service ALL my Jeeps and I got the Mystik for 4bucks a QT in a case of 12. I can get some 85w140, but the pour point on that is 0*F and in the winter it gets frequently below 20* here...I do not want to have to worry nor do I want to have to stock multiple weights of gear oil for each season... Knowing all this, if it was your rig, and you rarely wheeled it and had 31's on it, u think 80w90 dino would be fine as long as it had the limited slip additive in it? thanks again!
 
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