1981 240d differential fluid change question(s)
 

Hello, after searching and researching the many posts about this, it appears the consensus is to use Mobil 1 75 w 90 synthetic, so I changed it with that last week.

Now, differential seems louder.... has anyone else experienced this? any suggestions? go to dealer and get the hypoid oil? do it again, with heavier?

History: I bought the car about a year ago with 125,000 miles, daily driver, now has 133,000.. appears had never been changed before, so I thought it would be a good idea. manual transmission, fwiw.

thanks

The issue I caution about using synthetic oil, is that it will clean and loosen all the dirt and grime inside a diff. Especially on one that has never been changed. So what ends up happening since there is no filter to filter out this dirt and grime, it then gets recirculated into vital wearing surfaces.
Hopefully no damage is done, but I would drain and refill the diff with conventional 80w90 hypoid gear oil from a MB dealership.

Really? It is all in the mind.

I use the cheapest gear oil and never have a problem or hear louder noises. Also I do not buy the theory that it will loosen all the dirt and grime inside a diff. This is not a wrist watch mechanism. The gear is BIG. Have a beer and relax.

Cheapest gear oil will work wonderfully on a dirty diff.

But synthetic oil in a dirty diff is a bad thing. I've seen this happen several times. I never blindly install synthetic oil in a diff that used conventional oil all it's life. I have to open it up first, look inside to see the condition and clean it out if necessary. Then install the synthetic oil.

I like my cheap gear oil and my dirty diff.
 

I don't have problem with members using synthetic oil. It is their money and they can spend it anyway they want. I can clean the diff all day, it will be dirty by the time I go round the block once. So I never bother.

I disagree
 

I have followed the MB technical bulletin, installing synthetic 75w90 in thousands of vehicles with no issue.

My personal vehicles have synthetic 75w90, with no issues.

As a side note, I have installed conventional 80w90 hypoid gear oil for many customers with only minor increase of gear noise.


The only reason MB suggested synthetic 75w90 was lower gear noise.

This is a personal decision.

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I have no problems with using synthetic 75w90, in fact I run it in my vehicles. Both Mobil 1 and Lubro Molly with not a peep of gear or bearing noises.

What I've experienced and what I know is that differentials do get dirty with conventional fluid, especially units that have not been changed out for 100,000 miles. Now are you telling me you are not going see crap inside a diff with 100,000 mile conventional fluid? And are you going to tell me the detergents of a synthetic fluid will not loosen it all up? We agree to disagree.


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Unscrew the fill plug....and top off the oil...with the plug removed still....lower the car down....slide under the car....put your finger in the hole and feel if the oil is to the hole...

My car has never seen a diff oil change....I drained it and filled it with synthetic...it got a little loud....I topped it off and all has been fine....granted I would like to flush it out one more time...

Answer
 

I disagree with the negative assumption.
From my experience, most owners are wise enough to open and clean the differential when the old fluid comes out with sediment.

Often this is discovered and cleaned while changing the older clip style axles.

On vehicles new to me, I usually replace the differential mount and open the differential for cleaning at the earliest moment.


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I have a 1983 300TD which has a little over a quarter million on it. I recently dropped the diff while replacing the subframe mounts. I cleaned the crap out of the diff when I took it off. I have yet to refill the diff with fluid as the car is not yet driving.

Are you suggesting that convential oil OR running synthetic with an additional change is better than just switching to synthetic outright? I'd like to know what type of "crap" you're talking about. I'm pretty sure 90% of that "crap" is removed when the diff cover is removed and the diff is wiped clean. I do tend to agree that if it's just a drain and fill that plenty of debris will remain and another change (synthetic or conventional) will need to be done shortly.


Back to my TD. I'm planning on putting 75w90 LubroMoly in it. Since it's not that expensive or that inconvenient, would it benefit the discussion if I filled it with something different? Would another change of the oil in say 1,000 miles show anything in your opinion?

I'm pretty sure changing the oil with the cover removed to wipe out debris will be more beneficial than the choice of replacement oil. But I'm kinda curious as to what difference synthetic oils will make.

Repeating post# 6
 

The only reason MB suggested synthetic 75w90 was lower gear noise.

I have followed the MB technical bulletin, installing synthetic 75w90 in thousands of vehicles with no issue.

My personal vehicles have synthetic 75w90, with no issues.

As a side note, I have installed conventional 80w90 hypoid gear oil for many customers with only minor increase of gear noise.


The only reason MB suggested synthetic 75w90 was lower gear noise.

This is a personal decision.

.

I completely understand what you're saying, and I agree. I've put 75w90 LubroMoly in my other vehicles and have encountered ZERO issues.

I was simply giving DeliveryValve a platform, a willing test participant to test his theory.

OK
 

Understood.

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I do have to apologize to the OP for the negativity of my post. I didn't mean to make this situation a worst case scenario.

In the quotes above is what I have been trying to convey.



Absolutely when you are not opening up the cover to clean it out. I would agree that 90% of the debris can be removed by a wipe down. I would also consider another drain and refill after a couple hundred for peace of mind, but probably not necessary.

I've seen all kinds of debris. Metal shavings, dirt/grime/sediment, thickened fluid that leaves a grimy dark coating all over, burnt fluid, flakes of god knows what, water and etc.

As far as using something other than Liqui Moly or Lubro Moly 75w90? Yeah sure, try something else out.
I'm actually interested in this Service Bulletin Roy brings up in giving the OK to use 75w90. The original MB recommendation is to use 80w90. Food for thought....If you look at the MSDS of these fluids you will find the viscosity (cST) at 40°C between the two weights are significant. For instance Castrol Axle LS and Fuchs 80w90 has a cST of 140

Lubro Moly 75w90 has a cST of 82

Mobil 1 75w 90 has a cST of 103


So what does these numbers tell us? Lubro moly 75w90 is 59% the viscosity of the castrol and Fuchs 80w90. And Mobil 1 75w90 is 75% the viscosity of 80w90. Although I've had no problems, it would seem 75w90 might be too thin.