Gear oil preference

[Jeeper Creeper]

Im changing the differential oils in my front n rear. I want to use "Royal Purple" but Im running Detroit in both and I heard that you shouldn't use Synthetic oil with the Detroit. I also was wondering if I should use 75w -90w or 75w-140.
thanks in advance.

[jonah]

I have a detroit and I have been running the basic 75-90 from my FLAPS, autozone house brand for about 6 years. Never had any trouble.

Side note, in my SM465 I have tried a bunch of different oils to try and improve shifting. Started with the basic 75-90 gear oil, tried some fancy synthetic high speed expensive stuff, went to pennzoil synchromesh. I have to say, I could never feel much difference. I think the pennzoil synchromesh makes it a touch louder (but hard to tell in the jeep) and maybe cut down the warm up time a little, but that gearbox doesn't like to shift right till its good and hot.

[Jeeper Creeper]

I have a detroit and I have been running the basic 75-90 from my FLAPS, autozone house brand for about 6 years. Never had any trouble.

Side note, in my SM465 I have tried a bunch of different oils to try and improve shifting. Started with the basic 75-90 gear oil, tried some fancy synthetic high speed expensive stuff, went to pennzoil synchromesh. I have to say, I could never feel much difference. I think the pennzoil synchromesh makes it a touch louder (but hard to tell in the jeep) and maybe cut down the warm up time a little, but that gearbox doesn't like to shift right till its good and hot.

Ok think Im going to opt out on the synthetic and go with a name brand oil. I use royal purple as an engine oil, trans, power steering but since detriot dosent recommend synthetic I'll go with their advise.

[jonah]

Sounds like a good plan! I am curious why they don't recommend the synthetic, if the oil meets GL-5 specs it should be fine.

[Jeeper Creeper]

Sounds like a good plan! I am curious why they don't recommend the synthetic, if the oil meets GL-5 specs it should be fine.

I haven't talked to any reps for Detroit I'm simply going by the manual, and a few comments made on other forums.

[jonah]

Yeah, I was just curious. I poked around the internet a bit and didn't see any answers. Detroits are pretty simple, so it is hard to imagine that it would make much difference. But if they say use conventional, that is what I would do.

I did see that a lot of people use 85-140w.

[chad1]

I use synthetic 75W-90. If your rig has a tow package, and you are planning to tow I recommend 75W-140 in the rear.

[ROBERTK]

I recommend to everyone I do a regear for, Blue Bottle Valvoline. It works well for Every application. You can go 80-90 or the 75-140(I think thats the other) if you really need heavier for towing. And its not very $$.