rajsid

Last year I filled my 69's diff and tranny with SYNTHETIC 90Wt gear oil with a bottle of GM diff additive in the diff. I am going to change it to regular 90Wt gear oil.

How do I make sure the old synthetic stuff is all out before I put in the regular gear oil?

Is it ok to have residual synthetic oil in it when I add the new oil?

0grandmastercorvette

The only way to get ALL of the oil out of the diff is to drop it out of the car and remove the rear end cover. You can try to siphon out as much as you can, and hope for the best. I professionally have not tried to use synthetic oil in a diff or tranny. I use what was used back when it was designed. EXCEPT for the modified additive that GM sells now for the posi units. I would have to talk to a tech at the oil company and see what the properties for the oil are and if it is compatible. You may be surprised in what you find out.

"DUB"

rajsid

Thank you for your help.
A local corvette mechanic talked me into the synthetic crap. I should not have listened to him.

I don't even know what brand of synthetic he used.

Clams Canino

What specific trouble did the synthetic cause you?

-W (who was just given a free case of Royal Purple sysnthetic)

rajsid

Clunking noise from the diff. I fell that the manual trans is harder than it should be. First I though the clutch was wearing out, but the clutch is good.

0grandmastercorvette

Guys,
I didn't say synthetic was crap or wrong. I wrote that I don't use it( in diffs). BUT....if you talk to Royal Purple tech dept or another oil companies tech dept and they can give you information on the bentfits without causing any problems, then the facts are what they are, FACTS. If they can not give you definate answers, than that should tell you something.

I repair Vettes for a living and will not "guinnea pig" on a customers car and try something out UNLESS I have done EXTENSIVE research if I am changing to something different. The rear diffs are enough of a pain to deal with and I don't want to warranty anythiing because of changing to a "new" oil when I have been doing them for 20+ years and have had no problems.

Give them a call and you may get an education and then be able to express your new found information to others in the forum who may need it, when needed, like many of us do from time to time.
"DUB"

Clams Canino

I'm not sure that *calling* them would get me more than a positive spin sales pitch.
Here's what the Royal purple website says exactly:

Max-Gear is an ultra-tough automotive gear oil. It’s designed to maximize power and provide unsurpassed protection to heavily loaded gears. It makes gears run smoother, quieter, cooler and longer without overhauls.

Max-Gear outperforms other gear oils because it combines the highest quality synthetic oils with Royal Purple’s proprietary Synslide additive technology.

Synthetic oils enable Royal Purple to make superior lubricants, but it is Royal Purple's advanced Synslide additive technology that gives Royal Puprle's EP lubricants their amazing performance advantages. Synslide additive technology truly is beyond synthetic.

Synslide additive technology, Royal Purple's tough, EP lubricating film, provides maximum protection under boundary lubrication conditions typically caused by heavily loaded, slow speed and / or shock load conditions. This tenacious, slippery film significantly improves lubrication and reduces wear by increasing the oil film thickness and toughness, which helps to prevent metal-to-metal contact in gears and bearings.

Synslide additive technology is noncorrosive to gears and bearings, including case-hardened gears that are easily pitted by conventional sulfur-phosphorus EP oils. Synslide additive technology displaces water from metal surfaces and excels in protecting equipment in wet environments. It also fortifies the oil against the detrimental effects of heat, which causes oil to oxidize.

Max-Gear is recommended for use in truck, motor home / RV, and automotive front or rear differentials, manual transmissions, and lower gear units of marine engines that specify use of an API GL-5 or GL-4 fluid. All viscosities of Max-Gear are formulated with hypoid friction modifiers necessary for use in clutch or cone type differentials. No additional additives are necessary.


That's what THEY say anyway....

-W

thegazman

Raj...I started getting a bearing noise in my differential. I installed Royal Purple systhetic and the noise disappeared. (who knows). I also purchased and installed a kit that provides a plug in the bottom of the differential. Sure makes it easy to change the gear oil.

Paul L

Just curious. What is the modern GM equivalent of that old sperm whale oil. And that is what it was.

rajsid

Kenneth,
Where did you buy the drain plug kit from? Is it easy to install?

rajsid

Sorry, I should not have said it's crap. Let's just say that I am just looking for something that meets my need. I am a little frustrated because I got talked into it by a vette mechanic and I did not have any diff sound issues before.

I have to agree to this one. I can only guess what answer I will get if I call zMax and ask them if their product would be good for my car.

temvette72

I have heard that synthetics will make seals leak. In my personal experience, my diff was leaking a little. I changed to synthetic and it started leaking ALOT. Maybe it was a coincidence. I have since replaced all the seals and went back to conventional oil and posi additive.

Temvette72

73, Dark Blue 454

Our old Vettes have Eaton limited slip carriers in the dfferential and Eaton strongly recommends agaisnt the use of synthetics:

http://www.eaton.com/EatonCom/Produc.../FAQ/CT_128414
(See "Eaton Posi" section in the link above)

FWIW, Auburn and Dana/Spicer recommend against synthetics for their limited-slip carriers as well:

http://www.auburngear.com/aftermarke...6pagesize%3d20
(click on "limited slip" then see page 2)

http://www2.dana.com/pdf/5323.pdf
(See page 3 of the service manual attached above)

Clams Canino

That's what I was looking for - nor just anecdotal evidence but some concrete "facts" as it were.

-W

jb78L-82

I would bet that any issues folks are having with their differentials is NOT because they are using a particular type of gear oil but are more endemic to a problem with the differential to begin with. I am not surprised that Eaton would not recommend a synthetic for their differentials since these units were never tested with a synthetic when they were manufactured and no manufacturer is going to make such recommendations without thorough testing which does not mean that a synthetic is bad for a diff. That would be analogous to calling GM and asking if I should run a synthetic motor oil in my 78 when such oils were never tested when these motors were new-They are just not going to do it! I can say that I have used a synthetic gear oil in my 78 since the late 80's with zero issues-no leaking, no chattering, no failures (yet!)-just some real world experience. You can play it safe with conventional oil and the additive or use the most modern oils out there if you feel that they offer better wear protection-either way make sure that you change the oil at a reasonable interval based on your driving and you should be safe.

GTR1999

To make it real simple, use GM posi additive and Lucas 90 ot 85-140 gear oil. There is no need for anything else. If you still have issues with the diff then oil is not going to make a diffference.
I tap the housing for a drain on everyone I rebuild. I have seen "kits" sold that are nothing more then a hole saw and pipe tap to cut into the rear cover. I can't say I would recommend that approach or location but what ever floats your boat will work.

Clams Canino

Yes... but Eaton is still making/selling new units. You would think they would get around to testing synthetics at some point in time - no?

-W

08vycpe

I have synthetic gear lube in the rear of my '79 along with the GM limited slip additive with no issues. I don't think most peoples' issues with rear end problems have anything to do with the synthetic oil. It's probably more about the fact that the car is 30+ years old.

I also installed a drain plug. It's a 1/8" NPT tapped into a thick wall portion of the differential (not the cover!).

7T1vette

There is no problem mixing a bit of synthetic gear lube with 'standard' gear lube. They are NOT incompatible [with each other] and the residual synthetic will not cause you problems. The posi-trac has clutch plates that are designed to slip when you make turns (one wheel is turning faster than the other). Those plates will have different slip behavior when the lubricity of the lube is changed. The synthetic lube has different (better) lubricity and so your posi acts differently with it. Just suck out what you can with a cheap bulb/syphon pump and re-fill it with new lube and a tube of posi-trac additive.

pws69

This is all DARK AGES nonsense. Synthetics work great unless there is an issue with the gear set/bearings. You just need to make sure you add the POSI fluid.