Engine/Transmission (1998.5 - 2002) Synthetic gear lube question

[Texas Diesel]

I'm changing rear diff oil in my wifes truck, 1995 & 175,000, was just about to dump in Mobile 1 75W-90 synthetic when I remembered a thread bout someone who switched to synthetic and had a subsequent leak. It seems the thread went something about synthetic being a smaller molecule (!) and being able to get past old, tired seals. Any ideas appreciated. TIA

 

[Texas Diesel]

anyone?

 

[chuck3]

With that kind of miles you might be better off not going with the synthetic. Just my 2cts

 

[LSMITH]

If it is not leaking at all now it should be fine. It is small leaks that can get worse with a synthetic in my experience. Be sure you get the diff full or you will have seal trouble.

 

[Texas Diesel]

decisions, decisions. I tend to agree that synthetic may be asking for trouble, I have no drips but there is one area that has dust/oil buildup. Back to Dino guts I guess. Thanks for your input. BTW, the owners manual suggests to fill 1/4 to 1/2 inch below the fill plug, I have always filled to the point of running out.

 

[Bill_L]

When I did the gear oil change after breaking in my new gears I switched to synthetic. I also used gaskets instead of any kind of sealer.



No problems at all. Everything is great.



Bill

 

[LandShark]

Texas Diesel: I switched the OEM fluid at about 60,000 miles to Mobile 1 75w-90 with no problems. It's now changed about every 25,000 miles for the rear and every 50,000 for the front.

 

[RJOL]

I'm going to put Royal Purple Synthetic in mine when I change oil.



http://www.synerlec.com/specf_itm00005.htm



It has good specs, is formulated for positraction and sells for about $7-8 a quart. Anyone have experience with this product??

 

[Don M]

RP is good quality oil. You wont be disappointed. It does have the built in friction modifier you need to keep the chatter down in the LS. NAPA generally has it for 8 bucks or so.



85/140 is what several guys use here.



Don~

 

[C Schomer]

I just paid $7. 22/qt for a case of 75w140.

 

[pwknapp]

I've run the Royal Purple hard with no problems. Yes on the 85-140 if you are doing much hauling. Thinking of trying the LE ?# because I've heard good about it.

 

[HEYBOSS]

Originally posted by Texas Diesel

decisions, decisions. I tend to agree that synthetic may be asking for trouble, I have no drips but there is one area that has dust/oil buildup. Back to Dino guts I guess. Thanks for your input. BTW, the owners manual suggests to fill 1/4 to 1/2 inch below the fill plug, I have always filled to the point of running out.

That is where you can run into trouble. Never fill it to the point of running out. That's when seals start leaking because of overfill.

I just told this by reputable dealer and mechanics. I'm also in the auto repair business and we never fill them till running out. Go by the owner's manual.

 

[Vaughn MacKenzie]

Texas D, how about considering LE oil? It is spendy but is dino based yet quality stuff. Should give you something a step above the typical dino stuff without the leak worries.



I am running Amsoil 75W90 on mine, I just changed it today after 40,000 miles. It was still clear and little contaminants. There was barely enough metallic sludge on the magnet to wipe your finger on. It seemed to perform better than the Mobil 1 gear lube but the truck was still pretty new then.



The Amsoil has friction modifier in it but I had to add a little.



Vaughn

 

[gmorrison]

If you bought a new Pete or KW the warranty on the rear ends would be 100,000 miles. If you have a receipt showing you put in Mobil synthetic 75W-90 the warranty is automatically extended to 770,000 miles with both Eaton and Rockwell. If both Eaton and Rockwell put that much faith into the performance difference that synthetic gear oil makes in hypoid gear life, it would seem to follow that it becomes a no-brainer for our comparatively lightly loaded rears. .

Also, Eaton/Rockwell recommend a 200,000 drain interval with Mobil 1. LE mineral gear oil is not on Eaton/Rockwell's listing, BTW.

The synthetic 75W-90 will provide a cooler running rear end, improved fuel mileage or power and a longer lasting rear end. Also, the Mobil 1 75W90 has the limited slip additive included in the formulation.

George Morrison, STLE CLS

 

[John]

Texas Diesel, leaks are NOT CAUSED by the use of synthetic lubricants. Instead, when changing to synthetics after extended use with mineral based lubricants, false seals (i. e. sludge) are eroded away by the superior detergency of the synthetics.



I have used Red Line 75W-90, Red Line 75W-140NS, Amsoil Series 2000 75W-90, Amsoil Series 2000 75W-140, and NEO 75W-90 100% synthetic axle lubricants with excellent results.



Also, Royal Purple makes very high quality 100% synthetic axle lubricants although, I have never personally tried them yet.

 

[mimprevento]

Synthetics

I have a 95 Tahoe 4X4 with 3. 73 limited slip and towing package- small block TBI 350- I bought from my partner and he had done horrible maintenance when I got it at 36k miles 106,000 miles ago- I have run Mobil 1 synthetics in motor, both pumpkins, transfer case as well as transmission- my research indicates that synthetics actually reduce leakage in certain applications by better lubricating parts such as valve seals causing them to swell - I would think they would be kind to gaskets unless they needed replacement- I think my truck is a rolling test bed because my 350 has none of the valve seal leak by that haunts this otherwise great motor and causes some oil consumption at start up-I add oil between 4k oil change intervals rarely. No leaks in the other components- I have used Red Line as well in other vehicles but had to put limited slip additive in my Impala ss rear because the additive in the lube wasn't letting the clutches work. I would use synthetics if it was economically feasible and replace gaskets where appropriate. As soon as Big Green breaks in I am going to Delvac.

 

[Texas Diesel]

Update.



Mobile 1 went in at 170,000, now has 190,000 and no leaks. Perhaps we just got lucky or perhaps because we have changed it anually since it was new