Rear end overhaul - fluid question

[cpdenton]

I am about to go through the rear end in my 92 dually. I have a 4.10 open differential. I have the scotseal xl seals for both sides and a mag hytec rear end cover to go on it. I am going to rebuild all the rear brakes as well as one side of mine are now oil soaked.

Question is.....Is synthetic fluid worth it in the rear differential? With the mag hytec cover, the differential will hold 6 quarts, so that would be a large investment. What weight and style do you run in your truck?

 

[typ4]

I run syn, but I have a limited slip. if changine gear set break it in on regular then switch to syn after 5k. if not ,,,your choice. We have buckets of it for truck diffs at work so its easier. 75 90 I think.

 

[cpdenton]

I don't think I need to change the gearset at this point, but some 3.54 gears would be nice......

I am leaning towards conventional, with the extra capacity and cooling fins, it should help control heat and not break down as fast, right?

 

[typ4]

Correct. Unless you tow heavy it will be great.
That cover makes a huge difference, 30 degrees on my 4x4 towing 16k

 

[SLC97SR5]

I run full synthetic Valvoline 85/140 with a factory cover. Ford specifies a 100k change interval so for me the initial investment in synthetic is justified.

 

[Dieselcrawler]

royal purple full synthetic gear oil with a factory ford finned cover.

you can keep your expensive mag-hytec cover... lol

 

[cpdenton]

Yep, the cover was expensive...grabbed it from pick and pull for 9 bucks. No way I would have paid full retail for one.

 

[Ironman03R]

75/140 synthetic here. I went that route for longer service life, as stated above Ford recomends 100K service with that oil. I also have a limited slip.

 

[drinkypoo]

I'm running Lucas oil in my 3.55 limited slip. I think the important thing is to make sure it doesn't leak out, and not so much what kind of oil it is. Every single diff I've seen go bad has been run dry

 

[kendawg69a]

Have a stock 94 f350 7.3 idit 4x4 with 3.55s put on at factory...approaching 120k miles what would be recommended? Lucas or royal and what improvements can I get for a new diff cover?

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[drinkypoo]

Every manufacturer that I'm aware of now recommends synthetic in their differentials. I think Ford even suggested synthetic for Sterlings not long after the IDI. With that said, I just don't see any reason to spend a lot of money. It was cheaper for me to buy the Lucas (which was in stock) than the synthetic, and either will work in the limited slip. So I bought the Lucas.

 

[chris142]

Fyi.some of the aftermarket gear manufacturers dont recomend synthetic oil with there gears. They claim that syn does not remove heat like a conventional oil does. Personally i find that hard to believe especially since so many trucks come with syn from the factory

 

[drinkypoo]

Fyi.some of the aftermarket gear manufacturers dont recomend synthetic oil with there gears. They claim that syn does not remove heat like a conventional oil does. Personally i find that hard to believe especially since so many trucks come with syn from the factory

Yeah, that set off my BS detector so I whipped out google and from what I can tell there is no appreciable difference in heat transfer with synthetics at all, but there tends to be less friction and thus less heat produced which can only be a good thing, especially since synthetic is usually better-behaved at all temperatures than conventional.

If there were no price difference, I would run synthetic. But I had just replaced an axle seal and the diff cover and wasn't sure the stuff would stay in

 

[kendawg69a]

So ur not worried about synthetic performance but whether it'll stay sealed in the diff housing?

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[drinkypoo]

I don't want to leak synthetic because it's orders of magnitude more harmful than dino juice because of its stability. I think synthetic is better even where not specified, but not so much better that most people would ever notice. Only under the most severe loads like motorsport or extended uphill tows will the difference ever be relevant. I run synthetic fluid in any system that I'm sure isn't leaking or burning a lot of fluid, eventually. But my 300SD for example is on all dino as specified, and I see no reason to change it. My A8 specifies all-synthetic, including an incredibly specific transmission fluid, so that's what's in it. Well, and pouring out of it, but I just bought it and I'm doing the timing belt right now, that job includes several leak points

edit: Actually the A8 specifies normal brake fluid but I will change to Valvoline Synthetic DOT5.1 as soon as I get my hands on some.