2001 isuzu Trooper LSD oil

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any recommendations for the following.
GL-5 Limited Slip rear axle differential
Isuzu recommends 80W-90 up to 86F and 80W-140 over 100F (off the chart).
We had 31 days in the 90's this summer. Doesn't get below 10F

Royal purple Maxgear 75W-140 has the modifier already. Seems good but overkill and $21 / qt

I was thinking either
Mobil synthetic 75W-90
or
Lucas 80W-90

Also any recommendations on friction modifier?

Thanks

Alan
 
We have a 2001 Isuzu Trooper. I used Mobil 1 75W-140 gear lube in it. I went with the thicker gear oil in it because we go to our local farmer's market from May through October. We tow a 5x8 ft mesh trailer to carry the produce and vegetables. Also, since we use it mainly for the farmer's market and have a car for our daily driver, we don't need to drive the Trooper in the winter really, so, I wasn't too concerned about having a thicker gear lube in the cold temperatures.

It has been great so far, no issues that we can tell.

I used the Lubegard limited slip additive for the friction modifier. It has worked great. I tried it using the gear lube by itself, since that is what the oil companies recommend to do, then add an additive if needed. Experienced chattering going around corners, so, I added the Lubegard. Been quiet and smooth since.

Same thing with our Dad's truck. He has a 2006 Tundra. I tried changing the gear lube in the rear differential without adding any friction modifier. The truck was clunking and banging. Added the Lubegard additive. Nice and smooth and quiet, no more banging or clunking.

So, I really like the Lubegard additive.

Someone might say similar results are possible with any brand of friction modifier. I know CRC makes one that gets good reviews. I tried Amsoil Severe Gear lube and Slip-Lock additive a few years ago. It was nice and quiet for a while, but, then the Trooper started clunking and banging. The friction modifier wore out maybe? We were using it for towing. After that, that is when I went and tried the Mobil 1 gear oil and Lubegard additive. Thankfully, no issues for now.
 
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I also like that friction modifier and used it. I went into more detail on that and base oil selection on your cross-post at the Isuzu forum; you can check it there.
 
Going with Mobil 1 synthetic, 75W-90 and Lubegard friction modifier for the LSD rear diff. and the same oil in the front diff. I inherited this trooper and have no idea if and when the diff oil was changed. Alan
 
Rethinking after Oro comments and spending the time to read the comparison testing paper. I am going to use Valvoline High Performance 80W90 + Lubegard. I think it performs well in the area's of concern in the differential and it meets the price point. Since this trooper came with my wife, I have no idea if the differentials were ever serviced.
 
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