someone please tell me this guy is full of BS

[Mr.Scary]

Hey guys couple of quick questions i been wanting 2 change my differental fluid since ive owned tha truck and just havent got around 2 doin it until now. I went 2 tha parts house 2 pick up tha fluid and gasket. my first question is wut weight and brand have u guys used and wut do u recommend?? second question is when i was talking 2 tha guy trying 2 get my gasket i told him wut year make and model it was 4 and he told me he wouldnt change tha diff fluid if it were him cause tha fluid is most likely so old that changing tha fluid would only harm tha diff in tha long run or some nonsense like that and that ended up kinda makin me worry about doing it, so i didnt buy anything and figured id hold off until i got some opinions from u guys on whether or not i should change tha diff fluid. any comments or suggestions would help and thanks in advance guys.


p.s. someone please tell me tha guy was full of it 2
(it would help) LOL!!!!

[78 Chevyrado]

Never hurts to change diff fluid as far as I know. About the only thing I think new fluid could do is make some old seals leak a bit, though I've never had that problem with an axle.

Some people say not to change the fluid in an auto tranny if you wait too long, but I'm not gonna be convinced that putting new, clean fluid into a machine will do harm. There again, sometimes, it may make things leak, but I'd rather have the mechanicals lubed properly than keep the old gunk just to cover for a bad seal.

You're engine takes oil, just like the trans and diff; would you quit changing the engine oil just because you accidentally waited till 15,000 miles to do it?

[68 TT]

That is like saying it is too late to change your oil so you might as well keep on driving with the old stuff. Changing old gear oil in a differential will not hurt a thing.

I like to use Mobil 1 synthetic gear oil. I have seen improvements in power and gas mileage every time I have switched dino gear oil for synthetic. It only costs a little more than regular gear oil and you change it so infrequently that the extra cost is easily recovered by the efficiency improvement.

Changing the fluid in an automatic transmission will not hurt it either but doing a chemical flush on an old trans will sometimes make it fail soon after. The flush washes off the build up on the clutches and then the tolerances are too high and things burn up quickly. It just depends upon how much wear the trans had before the flush as to whether it will fail so you are better off not chemical flushing an old trans.

A filter change alone will still leave 2/3 of the old fluid in the trans. It will take more than 12-qts typically to fully change out the trans fluid. You can do a full fluid swap by disconnecting the tranny cooler output line at the front of the car and adding fluid as fast as it comes out until the fluid is clean coming out. Do this after installing a new filter and cleaning out the pan & adding 4-qts of fluid to the trans.

[Scootin 81 C20]

I would say he is full of it.

I'm running NAPA brand gear lube in mine.

[Mr.Scary]

78 chevyrado good point that makes a lot of sense. I think ur right. and on that note wut is a good brand and weight 2 use??? ive seen lucas and royal purple there b4, but i would also like 2 have some more input on weather or not i should change tha diff fluid, it doesnt hurt 2 get 2nd opinions. Anyone, anyone???????

[N2TRUX]

He's full of it as everyone has mentioned. One other thing is you need to know if you have a limited slip. If so, you will need to add the correct additive to the gear oil.

[Mr.Scary]

thanks ken i knew i would hear from u sooner or later. could u or anyone else out there please tell me how i would go about finding out if i have limited slip or not?? as far as i can tell tha truck is just tha bare bones stock 10 bolt no frills diff on it.

[powerdriver 1958]

Most older trucks run 90wt, So anything that said 80-90 or 85-90 will work.Might drive it around a bit before you change it to warm it up a bit.If its cool out that stuff is fairly thick and stinks.

[68 TT]

Clutch type limited slips are not that common in trucks so the only way to know for sure is to pull the cover off the diff and look at it.

If you do have a clutch type limited slip then use the gear oil designed for it with the proper additives. If you have a mechanical type limited slip like the GM gov-lock or a locker then any hypoid gear oil of the right weight will work. You can use limited slip gear oil in it without hurting anything.

[Mr.Scary]

LOL!! that dont sound like a bad idea.

[Mr.Scary]

so i can just go ahead and use tha limited slip additive wit out hurting anything? cause i dont really kno much bout axel's or rearends 4 that matter, as a matter of fact i didnt even kno there was a special additive u had 2 add 4 limited silp diff's i thought u just buy tha gasket and tha diff fluid and drain tha old stuff out pop tha gasket and cover back on and add tha fluid back. maybe i should do a bit more researching on this b4 i change tha fluid??????

[68 TT]

Just get four quarts of synthetic 80W90 hypoid limited slip gear oil and you will be good to go no matter what you have. If you end up not needing the fourth quart (depending upon which rear end you have) take it back.

You don't need a cover gasket just a bead of silicone will do just fine. Just clean everything on the mating surfaces of the housing and cover really well. I like the ultra black RTV. Let it cure for at least a day before you put any fluid back into it so you don't get a leak.

[3r!c84]

Originally Posted by Mr.Scary

so i can just go ahead and use tha limited slip additive wit out hurting anything? cause i dont really kno much bout axel's or rearends 4 that matter, as a matter of fact i didnt even kno there was a special additive u had 2 add 4 limited silp diff's i thought u just buy tha gasket and tha diff fluid and drain tha old stuff out pop tha gasket and cover back on and add tha fluid back. maybe i should do a bit more researching on this b4 i change tha fluid??????

wait you didnt have to buy a gasket. just get some RTV Silicone

[hatzie]

Originally Posted by Mr.Scary

so i can just go ahead and use tha limited slip additive wit out hurting anything? cause i dont really kno much bout axel's or rearends 4 that matter, as a matter of fact i didnt even kno there was a special additive u had 2 add 4 limited silp diff's i thought u just buy tha gasket and tha diff fluid and drain tha old stuff out pop tha gasket and cover back on and add tha fluid back. maybe i should do a bit more researching on this b4 i change tha fluid??????

The Limited Slip Additive will not hurt a non Limited Slip axle. I just use it when I'm not sure what I have.

I use Synthetic 75W-90 and a bottle of GM Performance #1052358 Limited Slip Lubricant Additive. The little bottle should be 10-15 bucks right from your GM dealer.
I've heard that with synthetic oil you don't need additive but I don't believe it.

The RPO code for limited slip is G80 ( I don't think G85 & G86 apply to our trucks). Should be on your option tag in the glovebox. If it's still there... and if you still have the original axle.

[cableguy0]

the synthetic oil already has the additive in it. you do not need the additive its a waste of money

[hatzie]

Originally Posted by cableguy0

the synthetic oil already has the additive in it. you do not need the additive its a waste of money

I'd check the label. I don't think it said it had limited slip additive.

[87 STEPPER]

Im guessin you changed the diff fluid and you allready know the guy didn't know what he was talkin about. But i just wanted to tell ya i use wall marts brand called super tech W85-90 it's the same thing as any big name brand but cheaper! I also use there Oil, ATF,Break fluid,PS steering, pretty much everything they have is produced by one of the big brand guys for them.

Also FWIW changing the fluid in a high milage auto tran's that hasn't had regular service through out it's life WILL cause the trans to fail i know it's happened to me!

[78 Chevyrado]

I put this in mine, no limited slip...

http://www.lucasoil.com/products/dis...id=12&loc=show

It's about the same price as the comprable valvolines, etc.


Why the limited slip differentials are different is the fact they have actual clutches in the diff case. the friction modifier/additives make change the oil so that the clutches will slip properly instead of chattering while turning.

If you have an open (regular, one wheel only spins when you peel-out) it's like you said, open it, drain it, sop out the bottom, reinstall gasket/cover and fill it up.

[old Rusty C10]

`I am going to disagree with the posters on automatic transmission fluid. If your automatic transmission oil is burnt to where it smells and is black in color the new fluid will make the trans fail alot sooner than if you left it be
Ive seen this happen and had it happen to me several times over the years

[Mr.Scary]

I would like to thank everyone especialy 78 chevyrado and 68 TT 4 there help and input. I think my mind is made up and im ready 2 tackle tha job hopefully it wont b so hard, ive never done this b4. i think ima try some of tha fully sythetic mobile 1 that 68 TT was talkin about sounds pretty good. sooooooo i dont have 2 use a gasket then???? just silicone???

[68 TT]

Originally Posted by Mr.Scary

I would like to thank everyone especialy 78 chevyrado and 68 TT 4 there help and input. I think my mind is made up and im ready 2 tackle tha job hopefully it wont b so hard, ive never done this b4. i think ima try some of tha fully sythetic mobile 1 that 68 TT was talkin about sounds pretty good. sooooooo i dont have 2 use a gasket then???? just silicone???

I haven't used a gasket on a differential cover in more than 20 years and have never had a leak when I waited at least 24-hours for the sealant to cure.

I have pushed it a few times and filled it up five or six hours later and didn't have any leaks most of the time but sometimes they did. Do the job on Saturday morning and fill it up Sunday morning and you will be OK.

I know it has been pretty cold in Dallas the last few days so put a little heater under there and keep it above freezing for the first few hours if you can.

[Mr.Scary]

Thanks everyone 4 tha help its very much appreciated!!

[Fastest32]

My advice Is never take mechanical advice from a parts guy. I changed mine when I bought it and it looked like chocolate ice cream. I used the mobile one synthetic and had no problems. I would also use a gasket just cause it looks cleaner and it's easier to clean up the next time. Plus they're pretty cheap.

[78 Chevyrado]

I have used rtv and gaskets on them. I used rtv on my dad's truck cuz that's what he wanted, but I used a gasket on my truck simply because I needed to put it together and drive it as soon as I was finished. I haven't had one leak wichever way you go.

If you use a gasket, tighten it to the proper torque specs. If you use RTV, follow the directions on the tube.

[cableguy0]

just use rtv and synthetic fluid. gaskets leak a lot sooner than rtv. the best stuff to get is called "the right stuff" it cures a ton faster than the standard black rtv and seals better.