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Old 05-15-2011, 08:22 AM
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Switching to Synthetic Differential Fluid

One car has 96,000 miles and has never had the differential fluid changed. The other car has 228,000 miles, had the differential fluid changed with the regular stuff at 150,000 miles, and has a slow leak.

Do the cautions about switching from conventional engine oil to synthetic also apply here....that, if there is a leak or crud plugging a leak, synthetic will make matters worse? I'm partial to synthetics, but am fairly confident I don't want to mess with the headache of resealing a differential if I don't have to.

Thanks.

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  #2  
Old 05-15-2011, 08:32 AM
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I had a leaky differential before I switched to Mobil 1 synthetic gear oil. I changed the axles a year ago put the new axle seals on and when I closed up the diff cover I put a bead of RTV on it. Had been leaking for 3 years and this solved it! 1 year with synthetic gear oil and still no leaks...
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Old 05-15-2011, 10:22 AM
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There are no typical concerns. If a part shows a leak when syn oil is introduced, there was an issue there far earlier. Fix it!

Back around 2001 or 2002, there was a set of photos on here that showed the internals of a 300D diff on regular fluid and how running M1 cleaned it up substantially over a few thousand miles. Good stuff.

I'd be running syn. M1, Amsoil SVG, Royal Purple or equivalent.
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Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)
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  #4  
Old 05-15-2011, 11:11 AM
Craig
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I just switched my 300D differential to synthetic. It needed to be replaced while swapping out a half-shaft (failed boot) and I don't see ant down-side in using synthetic. I think I'm also going to go with synthetic tranny fluid at the first service for the reman tranny I just had installed.
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Old 05-15-2011, 11:33 AM
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Quote:
Originally Posted by shertex View Post
One car has 96,000 miles and has never had the differential fluid changed. The other car has 228,000 miles, had the differential fluid changed with the regular stuff at 150,000 miles, and has a slow leak.

Do the cautions about switching from conventional engine oil to synthetic also apply here....that, if there is a leak or crud plugging a leak, synthetic will make matters worse? I'm partial to synthetics, but am fairly confident I don't want to mess with the headache of resealing a differential if I don't have to.

Thanks.
synthetic oil causing leaks is no longer a valid concern in fact just the opposite, unless you have a real gusher
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Old 05-15-2011, 01:08 PM
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When did MB start using synth gear oil? I know VW was using it by the 80s (Calling it "lifetime"), I'd imagine MB wasn't far behind.
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  #7  
Old 05-15-2011, 01:16 PM
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According to the FSM, my '96 was shipped without synthetic gear oil in the differential. I do not know where the cutoff was, but the FSM says that when replacing the gear oil, use the new synthetic in all cases.

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