advice: GM 10 bolt, worked hard - what oil?

[91l98z28]

I have a GM 10 bolt in a '94 1500 extended cab. 350, automatic. Pulls a ~ 4000lb boat/trailer combo in the summer (california valley heat) including hill climbs where I might be WOT for a minute or more at a time. It also sees quite a bit of highway miles at California highway speeds (75+) but is an aerodynamic brick, so that also loads the rear axle.

With 95,000 miles on it, I got tired of the howl from the rear end (been there since I bought it at 60,000 miles, previous owners pulled a trailer that was probably too heavy for the truck). So I'm having the 10 bolt rebuilt - new R&P, new bearings, etc. The shop doing the work says the drive side of the R&P is wasted.

To get to the point, I'm looking for recommendations on diff gear lube to make the new R&P last. I like the idea of redline shockproof, but I'm not sure this is the correct application (year round use, not racecar). I need something that will withstand sustained heat (hours on the highway at 75+, pulling the boat up into the mountains, etc.) and protect the gears. I'm willing to sacrifice MPG to make the gears last longer, and I don't care if the oil is a bit pricy or special.

So, any recommendations beyond synthetic 75w90 GL-5? Maybe something like amsoil Severe Gear 75W-110? I have no brand allegiances (I run PP in my miata, german castrol from my personal stash in my camaro, and Rotella 5w40 in the truck) so anything is fair game.

 

[Danno]

What does the shop doing the work recommend?
They must be offering some kind of warranty once they complete the job.

 

[EricF]

I would think the second you get the synthetic filled in, the heat and lube breakdown issues are nillified.
I wouldn't get into the shockproof oils, those are for high stress, short intervals usually.

As a fan of Lubegard products, I wouldn't even consider LG if synthetic were in the diff.

 

[SwampSurvivor]

I've heard people say that synthetic gear oil isn't ideal for heavy towing.

 

[91l98z28]

Warranty requires oil change at 500 miles in addition to the heat cycling break in process. I'll be doing the oil change at 500 miles hence the reason for this post. The shop fill is Valvoline 80W90 GL5, which is fine for the 500 mile break-in; but I want something better in there after that. I'm sure there is nothing wrong with the valvoline per se, but I'll take any extra protection I can get.

 

[CarbonSteel]

Jim Allen can provide some insight into this; you may want to PM him (or he may see this post and chime in).

 

[beanoil]

Delo ESI 85w140. Works great, not expensive, unique borate additive. It just works.

 

[Jim Allen]

Here I am.

Simple Answer: Any synthetic 75W140 or 75W110.

Complex Answer: Install a diff temp gauge. This will help steer you towards the right viscosity answer and offer you a way to monitor diff oil temps in heavy load situations so you can take steps to protect the diff as necessary. If you see low sustained temps, ie no higher than 225, you can use a synthetic 75W90 so that your solo fuel economy is optimized (parasitic drag from heavy gear oil is a significant MPG detractor). If you see sustained temps above 225, then a heavier oil is in order. Ideally, you want to maintain no less than 90 grade viscosity at any sustained temps. Granted, the additive package will protect even as the oil gets thinner, but lubricant viscosity is also protection in and of itself.

OR, the alternative is to control axle temps. A finned, high capacity cover (Mag-Hytec is the best IMO) will reduce oil temps. In my tests, it knocked a good 15-20 degrees off peak oil temps and the greater volume of oil helps hold the temps down over the short term, as well as adds to the OCI because of the larger volume.

IMO, anyone that tows a significant period of time should have a diff temp gauge. More so the closer to the limit you are working a particular axle. The Mag-Hytec cover has a port for a sender but you can weld a bung into any diff cover. Some people Screw senders into drain plugs but that makes them vulnerable to damage IMO. I had a diff temp gauge in a 10-bolt (diesel Blazer) and used an Autometer gauge (which says "Diff Temp") and one of their weld-in bungs. Now, I have an Isspro diff temp gauge in my F150 and an AUtommeter in the F250.

Look HERE for more info.

Some general things I have learned:

1) At a given load, a larger ring gear axle will run cooler than a smaller (more tooth contact to spread the load)

2) At a given load, a lower ratio (4.10. 4.56:1 etc) will run hotter than a higher (3.23-3.55:1, etc, ... more hypoid action creating more heat). The lower ratio has less tooth contact but higher contact pressure.

3) At the same viscosity a syn oil will run about 10 F cooler than a mineral, EXCEPT mineral oils with special additives. I used a LE oil (LE607), which was a straight 90 but one that had lots of a proprietary moly-like additive that surprised my by running as cool as a well-known syn. The Chevron ESI oils are said to exhibit similar characteristics, though I have not seen that demonstrated personally.

4) Fluid friction from an oil heavier than needed for the temperature causes the oil to run hotter.

5) At any given load, the faster you go, the hotter the diff runs. Some of that is load due to windage but some comes from the diff itself... that hypoind sliding action occurring more often and heating the oil faster.

6) A higher viscosity oil than you need for the temp range costs fuel economy.

 

[Doog]

Originally Posted By: 91l98z28
Warranty requires oil change at 500 miles in addition to the heat cycling break in process. I'll be doing the oil change at 500 miles hence the reason for this post. The shop fill is Valvoline 80W90 GL5, which is fine for the 500 mile break-in; but I want something better in there after that. I'm sure there is nothing wrong with the valvoline per se, but I'll take any extra protection I can get.


Do yourself a favor and do the first 100 miles in city stop and go. Then go on the highway. After 500 put Amsoil or Redline 75w140 in there.

 

[oldhp]

+1 on Jim Allen everything he said advice.

 

[147_Grain]

Totally agree with Jim Allen on checking the oil temps.

If it was MY vehicle, I'd see what temps I'm running and then use Amsoil 75W-90 or 75W-140 (depending on how hot the differential oil is running).

Pablo can provide assistance on this as well!

 

[WishIhadatruck]

I run the Amsoil 75W-110 in my diff per Pablo's advice. I tow a 5600lb travel trailer and have pulled it about 8k miles over the last few years. I assume the rear gears are original, I bought the car with 10 years and 90k miles on it, and it now has about 171k miles on it.

Now that I think about it, I should probably check to see if mine needs changing.

 

[dnewton3]

I'd supplement Jim's advice with the following:

Your break in period is critical. No towing or heavy hauling of stuff, and only light to medium throttle for the first 500 miles; vary your speeds. Then dump fluid and change again. Next time run another 500 miles with moderate to heavy throttle occasional applications, but lots of speed variation (as much as practical). Then dump and fill again.

The first two OCIs are to flush out the machining and set-up and break in wear. Therefore you might as well use a good quality but inexpensive conventional oil. Keeps cost down and such. Any decent house brand of GL-5 will work. As you're not going to be doing anything heavy, an 80w-90 would work just fine.

Then after that second fill choose any decent GL-5 syn you wish, and run it for a long time. Use temps and UOAs to then judge your real OCI plan. Pick a vis appropriate to the majority of use you'll see.

If you tow heavy, but only do so about 5% of the time, then a 140 grade is a poor choice. If you tow heavy most of the time, a thicker grade might be more appropriate. Don't choose your fluid based upon a "worst case" scenario that you'll rarely ever be in. It's not like a 75w-90 syn won't do a good job; they will. The point is to predicate your selection based upon the largest percentage of use.

 

[dailydriver]

Originally Posted By: beanoil
Delo ESI 85w140. Works great, not expensive, unique borate additive. It just works.


I thought ONLY their 80W-90 used the borate add pack?