Question all,

I do not need to put additive in the gear oil unless it’s got a limited slip diff, correct?

 

Additive is for limited slip differential only. I recommend checking the differential breather while you're working on the axle. It is often overlooked and will cause seal failures if it doesn't vent properly. A word of caution regarding synthetic gear oil. Depending on the brand it can be very low viscosity compared to the original gear oil from the factory. Recently had a F150 with 1 axle seal leak. Replaced both seals and filled with fresh synthetic gear oil, breather was OK. One round trip to work (60 miles) and both axles and pinion seals started leaking. Replaced seals and went back to non-synthetic gear oil. No leaks so far.

 

Thank you for the information. This forum has been a wealth of information. I literally just bought this truck and it’s my first foray into the old 3/4 ton diesel world. I appreciate ya!

 

I agree, this forum is one of the best you can find. I read and followed it for years before I became a member.

 

X2 on checking and keeping the Diff vent open.
It`s actually on a rubber hose end up high as
I remember. Make sure where it connects to
the Diff. it is not cracked where it pushes on
to the little metal tube.
The plastic Breather thingy is pushed in the end
of the hose on the high end.

I drained the Diff. about a yr and half ago. Some
nasty looking crap came out, probably never
changed, IDK. Cleaned up the rusty Cover down
to shinny metal and POR`d it. Then filled with
Synthetic gear Oil. So far no leaks in almost 20K
miles. Also did the T-19 Gear Box. Pulled the 2
PTO side covers and resealed them.
Filled with Syn. Gear Oil also.

Used RTV think the black stuff, so far no leaks.\

I have read over the years Syn. oil will cause leaks.
Also read it will clean up crap that could be build
up on the seals. Maybe more so with engine seals?
and a leak will start. Maybe the old seals were ready
to go anyway?

With just over 300K on the 6.9, not going to switch to
Syn. Oil with the amount of oil it consumes.

I have a 1980 Mercedes 240D 4-spd. It uses around
a qt every 1000 to 1200 miles, and this is with over
300K miles.
At 322K I started using 15/40 Syn engine oil. Now it
is up to 383K miles. The oil useage has stayed the
same.
Have Syn. in the Eng. Trans and Diff, with no noticable
leaks.

Iam not selling or pushing Synthetic Oil, just my experience
with it.

Charlie

 

If you snap your hose off looking at it, I found fuel rated hose at napa and got 5 feet of it and ran it up by my taillight. I don't intend on driving through anything that crazy, but nothing is getting in up there on accident, and I have an easily accessible emergency stash of fuel rated hose

 

Interesting experience there, though I'm not sure synthetic would magically cause it to leak. While I have heard that about old engines with lots of miles, I've also heard that _modern_ engine oils with detergents will cause leaks, or loosen up sludge and clog oil pumps etc on old engines that never used detergent oils before. Regardless, thats a conversation best done on an oil discussion forum...
Synthetic gear oil also can help lower differential temperatures and improve mileage.

Some synthetics come with the limited slip additive already added (or don't need them due to the synthetic formula?). If you don't have a limited slip, it won't cause any problems having the additive. If you do have a limited slip, the additive is actually there to help it slip, too much additive and it will slip easily negating the effect of the limited slip.

 

I remember when that rumor spread like wildfire but turned out to be B.S. I switched to synthetic in my manual trans, transfer case and differential about 200,000 mile and 20 years ago and no leaks yet.

 

Like an old Wives Tale.

I used Synthetic in my 82 Datsun 720 for over 400K miles since it was new.
Helps in the cold weather, especially shifting the Transmission. (Gear Box)
Never owned an Auto. Trans. probably helps there too.

Using the proper additive like in the Posi Differential, the proper gear/oil lube
is important also.
I used Syn. gear oil in the Datsun 5-spd and shifted great.
When I got the MB 240D with a 4-spd Manual, it called for ATF.
If the Gear Oil worked in the Datsun why not in the MB?
As I remember (Too many yrs ago) it slowed the shifts down, and
difficult to shift. Or maybe some grinding.
Switched to a SYN. ATF and shifted like butter. Zoom........Zoom....

I know when I drained and added the Syn. Gear Oil to the T-19 is shifted
a lot smoother.


Charlie

 

Synthetic doesn't cause leaks - old wives tale. The superior properties of synthetic can be seem in big trucks. 99.9% of all big trucks come from the factory with syn in the trans and diffs. The warranty with syn in the diffs is 750K miles. If you run dinosaur oil the warranty is only 250K. That says something if Eaton, Rockwell & Meritor will stand behind their product 3x longer just because you run synthetic oil!!

 

Synthetic oil is thinner than it's dino counterpart, but better oil so it holds the same rating as a high viscosity conventional oil. That's where the less drag/better mpg when used in differentials comes into play. Also less friction but mainly because of the actual viscosity.

Kinda like how when you buy a 10 ply tire, it's not actually 10 plies, it's usually 6-7 but rated at 10 ply...

On most things it shouldn't cause leaks, but some things also weren't designed for it, and it will cause leaks, mainly due to wore parts/seals that should be replaced, but aren't wore enough to leak with conventional oils.

I get asked alot about rear main seal leaks on motors with high mileage, more than half the time it's because they've recently switched to a full synthetic oil, on a motor that wasn't designed for it, nor needs it. Not to mention the high milage, i tell em to switch back and save the money for something else..

Conventional with routine maintenance is just fine, however most don't do their own maintenance or don't want/don't have the time for the down time so synthetic is a good option for them. It also does wear less, but not if you don't keep on top of it's maintenance schedule...

I was given some ams oil 75/140 full synthetic, I put it in my trans (t19) and it was thinner than the 80/90 conventional that came out of it, shifts worse too, it's pretty wore though, I'll probably swap to a 75/140 conventional and put that oil in the rear diff, it's got a 500k service life iirc...

 

Right, and among others, one of the main benefits is that it doesn't degrade due to excessive heat. I used to tow heavy in severe summer heat, that's why I switched to syn in the gearboxes in my pickup.

 

The best way it was explained to me is:
Dino oil molecular structure: Open a box of uncooked spaghetti and spill it on the floor.
Synthetic oil molecular structure: Uncooked spaghetti in the box.