Which weight to use for rear end 75w90 or 75w140

[upsguy]

I'm planning on installing new rear diff cover this weekend and was trying to decide which weight of fluid to use. Manual says 75w140 for heavy duty towing.

Probably 30 percent of miles will be used towing less than 7,000 lbs. Is this heavy duty towing for this truck? It's no where close to the maximum tow weight. Also, with new cover, I'll have almost double the fluid as stock and cooling fins will help to keep rear end cooler. Would it be alright to use 75w90 under these conditions? What would it hurt to use 75w140 all the time anyway?

[mark45678]

fuel economy is the only thing you can gain on by going to lighter fluid. I have heard some claim MPG gains by going to full synthetic oils in the running gear .I have run the lighter 75/90 redline in my truck with no issues. I am not a oil expert like some on this site so it should be a good post....

[WILLYD-MAX]

I don't think you need the 75/140 with the weight you are pulling. Could I recommend Amsoil severe gear 75/90? I don't think you would need the new rear diff cover if you ran Amsoil. Just my 2cents.

[Docklobster]

Good questions - I was debating the same. Amsoil 90 or 140? I tow a 10,000lb trailer about 60% of the time. Stock everything.

[upsguy]

I don't think you need the 75/140 with the weight you are pulling. Could I recommend Amsoil severe gear 75/90? I don't think you would need the new rear diff cover if you ran Amsoil. Just my 2cents.

I probably don't need the new rear diff cover anyway, but in the long run I thought it may pay off. Anyway, I like being able to check the fluid level with the dip stick. I'm getting to where the easier something is, the more likely it is to get done. The dipstick has two lines on it, the lower one is the bottom of the axle and the top is the top of the axle.

I think I'll run the 75w90. Maybe I'll get a temp guage and monitor the temp while I'm towing. If I was real industrious, I'd go on one trip with each weight and track mileage and temp. But I'm getting ahead of myself, I've not even gotten the cover on yet!

If I can find the amsoil this weekend I'll use it.

[jbplock]

...If I can find the amsoil this weekend I'll use it ...

If you can't find the AMSOIL Mobil-1 75w90 is good too (AutoZone sells it) ... just be sure to use a Synthetic ... Also, if you haven't seen it, the following thread from the archive's is a good read on 2500/3500HD Rear Diffs ...

Eaton's Recommendations on Locking Rear Diff Service (http://www.thedieselpageforums.com/tdpforum/showthread.php?t=17448&highlight=txdoc) (3 pages)

:)

[Idle_Chatter]

Either weight is fine if you are not a hot-shot towing heavy all the time. If you are a continuous heavy puller you should use the 140. I have used both weights of Amsoil synthetic in my rear diff. I happend to have the 140 "on hand" because I had been encouraged to use it in my 1500 6.5TD Tahoe and went ahead and put it in my DMax. I ran it for a couple seasons from upstate New York winter to south Florida summer and had absolutely no issues with economy or driveability. When I drained the 140, it was still clear and amber and I could swear I had less metallic paste on the drain plug magnet, so it's apparent that my diff appreciated the extra lubrication. I tow rarely and light, so I've been running the lighter weight syn in both diffs for the last couple of changes.

[upsguy]

You can find mobil 1 synthetic gear oil a heck of a lot easier than you can find amsoil, so that's what I got. I've used mobil 1 synthetic motor oil for about the last ten years anyway.

[DA BIG ONE]

I had changed out my diff lube to heavy weight only to find that it would blow out the vent at speeds above 75 mph on a short 1.5 hour drive, went back to 75-90 no issues at speed.

There is a tech bulletin about front diffs in colder areas of operation that require the use of synthetic fluids only for proper bearing lube.

[Idle_Chatter]

I had changed out my diff lube to heavy weight only to find that it would blow out the vent at speeds above 75 mph on a short 1.5 hour drive, went back to 75-90 no issues at speed.

There is a tech bulletin about front diffs in colder areas of operation that require the use of synthetic fluids only for proper bearing lube.

The DMax rear diff is spec'd for synthetic lube only - and the heavier lube is recommended for heavy towing. I've never heard of a pumping problem with the rear diff on a Dmax - must be something unique to your Burb. There have been some pumping issues with the DMax front diffs, that was part of the reason for specifying the new white diff vent cap over the original 2001 black diff cap for synthetic lubes (white has a metal rotating vent hood internally and black is fixed). I've had syn lube in my front diff for over 100,000 miles on the black cap without a problem - I guess it just depends on where your black cap "lines up").