Rear Diff fluid question.

Okay, I did a search and I don't think there will be any problems, but here's the situation:

Its time for me to change the rear diff fluid in my Chrysler 8.25. No one had the non synthetic 75-90 fluid that it calls for so I picked up some Mobil One synthetic 75-90. Well, I started to install it but my father saw what I was doing (I have to do my work in his garage since I don't have one) and questioned whether it was a good idea to put synthetic fluid in it. I assured him that people do it all the time and it won't be an issue. Well he responded with "Yeah, but are you sure the two fluids are compatable? When you change out the fluid, you don't get all the old stuff out unless you pull the axles, etc."
I didn't have a good response, so I held off on doing it and thought I would ask around if it will be a problem or not. I'd rather not have to replace a perfectly good rearend.

So will it be a problem, are the two compatable?
TIA

Yeah you're fine. Only question: does the axle have trac lok? if so you need to go to the dealer and buy a $10 bottle of friction modifier.

Probably, but you can always take an ounce of each, put them in a glass jar and stir them a bit. See if they mix and then let it settle for awhile and see if they separate again. Synthetic/fossil oil blends are pretty common, but I'm not sure if they add emulsifiers (or a mixing agent) or not.
I've had some bad experiences with synthetics (in industrial applications) in wet clutch type gear boxes. So I avoid synthetics, with limited slip type diffs.
Other than that, I've had no noticeable problems. Though some seals seem to get spongier/softer/swell, when using synthetics.

When I checked my rear diff oil recently, it had a translucent gray color to it. And it didn't have the strong sulphur smell that normal gear oil has. Does this sound like it was filled with synthetic?

I was surprised in finding a rubber fill plug on mine. (96 8.25 Chrysler) Is there a retrofit plug available, or is one necessary?

ROE

I put synthetic into my Chrysler axle, just as a normal change without flushing. That was a long time ago, and it's fine. A little quieter, though not as much as I'd hoped, but still working at 252K miles.

Synthetic is usually compatible with dino oil. It would be a hard sell if it weren't.

Runnin'OnEmpty said:

When I checked my rear diff oil recently, it had a translucent gray color to it. And it didn't have the strong sulphur smell that normal gear oil has. Does this sound like it was filled with synthetic?

I was surprised in finding a rubber fill plug on mine. (96 8.25 Chrysler) Is there a retrofit plug available, or is one necessary?

ROE

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I dont know really. BUT Mobil1 75w90 synthetic reeks to high heaven

There are outfits (Castrol? Vavloline?) that sell 50/50 mineral oil and synthetic oil for engine use, and I see no reason why this shouldn't be the case with other lubricating oils.

Synthetic oils are designed to be the same as natural oils - only moreso (more sitction to metal, more lubricity, more resistance to operating temperatures, ...)

Given that I've also mixed synthetic and natural oils in the past without difficulty - except auto transmission fluids - I see no reason why you should have any trouble draining and replacing with synthetic. Eventually, the mineral oil will all "rinse" out - unless you reverse the process and replace the synthetic with mineral.

Oh - and be sure to check to see if you have LSD, and get friction modifier. By the time the clutches start chattering, it's already too late and you'll need to replace them...

5-90

Runnin'OnEmpty said:

When I checked my rear diff oil recently, it had a translucent gray color to it. And it didn't have the strong sulphur smell that normal gear oil has. Does this sound like it was filled with synthetic?

I was surprised in finding a rubber fill plug on mine. (96 8.25 Chrysler) Is there a retrofit plug available, or is one necessary?

ROE

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No it sounds like it's got water in it.

Cool, I didn't think it would be an issue but I wanted to check it out before hand.

Yes, I've heard of synthetic blends but I didn't think about that. Thanks for the help, I'll be doing it this weekend.

On a side note, IIRC my owners manual recommeds synthetic if you are going to be towing. So if you plan on towing a trailer just consider it preventitive maintenance. As for that rubber plug, I replaced mine when it developed a slow leak. Under $5 from the dealer and it hasn't leaked since. I've seen ways to retro a metal plug into it, but frankly I'm just too lazy

All the later DCX service info reccomends 75/140 synthetic for towing use, so it would be safe to assume that this would be a good option for other heavy duty use such as 'wheelin' too.

Just read the bottle, usually it will say "compatible with all other oils" or somthing like that.

Quote:
Originally Posted by Runnin'OnEmpty
When I checked my rear diff oil recently, it had a translucent gray color to it. And it didn't have the strong sulphur smell that normal gear oil has. Does this sound like it was filled with synthetic?
ROE

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Originally Posted by langer1

No it sounds like it's got water in it

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Thanks for the reply, Langer1. I checked both diffs, and the gear oil looked the same in both. It looked clean and clear when dripped on a paper towel, so I don't think it has water in it.

I just traded this summer for the 96, and was going to change the diff oil, but it looked so good, I think it's relatively fresh. It's just a different color and smell than what I was used to......

ROE