62Cadman

I have been tiring to decide which oil to use in my corvette 6 speed manual. In reading all the forums...I am totally confused, so I went out and bought 3 qts of Royal Purple 10w40 synthetic motor oil. Does anyone have any experience? My Corvette has 100,000 miles on it and I just paid $1700. For a new clutch pressure plate etc. and after it warms up there is a noticeable noise when the clutch is out and in neutral or driving in gear at mostt speeds. I know the shop that did the work didn't change or even check the transmission fluid. That's why I am changing it myself. Does anyone disagree with using Royal Purple synthetic 10w40?

dizwiz24

,I disagree.

put motor oil in your engine, not transmission.

you need gm or amsoil syncromesh

1985 Corvette

Pennzoil Synchromesh is readily available at any of the major auto parts stores. You'll need three bottles and a 17mm Allen head socket. Pick up a cheap $4-$6 pump to screw on top of the bottle (the brand is Performance Tool, sold at Autozone), so you can pump the fluid into the fill plug easily. After you drain the trans, clean the plug and apply a smear of permatex to a portion of the lower threads before threading back in. Make sure the fill plug can be loosened first before draining.

You'll probably see the threads on the plugs are jacked up where someone used vice grip pliers or channel locks on them. Halfway through bottle 3, you'll start to get synchromesh dribble out the fill hole, that's when you plug it. Audi dealerships have a replacement for the fill and drain plug but I forget the part number.

Cjunkie

Motul 10w60 there is your textbook answer.

RollaMo-LT4

Yeah, I disagree
I wouldn't use that in mine.

856SPEED

I have a feeling the noise is related to your new clutch; not the lubrication in the tranny. From what you are describing, you have a new aftermarket clutch which may or may not include a single mass FW; un sprung hub disc, etc. etc. Those things will cause the "noise" at idle w/o the clutch engaged as you describe.

Aside from that, you want to use a sychromesh lubrication; whatever brand you choose, it should have the word "synchromesh" on the can or bottle.

mickey5

GM sells manual synthetic syncromesh tranny oil for for the 6 speed you have. These transmissions will "growl and whine"...its just the nature of the beast. I took mine in when it was new and complained about the noise but was told it was normal. It sounds and shifts today after 27 years with one oil change at 50k just as it did then. You'll learn to love it.

Purple92

If you've spent some time reading the various threads on what oil to use in the ZF gearbox - you've probably realized there is no real consensus.

In a lot of ways - that's good news - if people were finding that gearboxes were failing if you use ******* that would be telling you something. The reality is the ZF box is a very tough gearbox fr street or road race applications - it doesn't like clutchless "bang shifting in drag racing too much.

Personally - I use Redline MTL, but there are quite a number of folks in the Amsoil camp, others in the Royal Purple camp - and some even like the Castrol 10W-60.

IMHO - any of the above will probably work just fine for you - use what you like. I would highly recommend changing the fluid after 100K miles, and while it's possible it might cut down on the noise - if you find the "noise" is worse when the gearbox is hot, and if it goes away when you shift to neutral - you're probably going to find that what oil you have in there is utterly unimportant as far as the noise goes.

scotth48

I use AMSOIL.

scotth48

I would think it is a clutch problem. If you replaced the dualmass flywheel with a lightweight one that would create the noise. There are other posts that talk about that and how to fix it. Do a search!

GREGGPENN

Taking the trans out w/o losing/changing the fluid would be a trick. To pull the trans, the driveshaft must come out...and without it inserted, fluid drains out the back. That's not saying they didn't keep most of it inside then add "something" back into it. If they can't clarify, I would change just for the sake of making sure you have one type of fluid (vs some mixture).

Though ZF used to recommend Castrol 10w-60 motor oil, I've used it and didn't like it. Without modifiers to improve synchros and shifting, It's probably a BAD idea. The amount of wear tested by the ZFDoc also seemed excessive (to me) but hey...the's the expert and didn't throw out a red flag. OTOH, his business is to take them and AND FIX THEM!

Amsoil is good. The GM replacement is probably fine too. Personally, I'd try MTL. All contain modifiers to IMPROVE shifting. And, I will tell you the reason I didn't like the Castrol 10w-60 motor oil was the less-then-ideal shifting.

I'd even try Royal Purple Manual Trans fluid (assuming they make it). Just don't put motor oil in it. Put that oil in your motor. If you live in an area where you'll drive in near/sub-zero temps, you'll also be happier if your MTL oil choice has a 5w "base" vs a 10w "base". IOW, 5w-30....which is what AMSOIL is.

Kubs

I would agree with the others and definitely NOT use engine oil in the transmission. Even though the spec for a ZF6 is a lighter weight lubricant, engine oils are not designed for the different metals and tight clearances in a transmission. The engine oil may not have the proper protective additives or sheer strength required.

I, like a few others, use AMSOIL Synchromesh. I have used it for the last 10 years or so (regular changes) both on and off the track and I have had zero issues.

https://www.amsoil.com/shop/by-produ...30/?zo=1934716

mickey5

motor oil can't be bad for the ZF considering thats what was put in originally. I changed the trans oil at 50K in my '90 and GM put back their synthetic manual trans oil. It operates and growls exactly like before with no problems. I hope it lasts another 50K as it does now.

DGXR

This. Synchromesh fluid is basically a lightweight gear oil. The factory fill for the ZF-6 was a Havoline 5w30 motor oil with additives for this specific application. Most manual transmissions of this era call for 75w90 or 75w140 gear oil. Heavier oil is not correlated to higher film strength, and good flow is important in this application so don't use anything too heavy in your ZF-6. Finally, manual transmissions chew the hell out of oil so even the best synthetic fluid won't last forever - it will require servicing once in a while. The owner's manual claims that the fluid does not require replacement but anyone familiar with lubricants knows this is ridiculous.

It's a safe bet to use a fluid equivalent to GM part numbers 1052931 or 12345349. Lots of synchromesh fluids compare to these specs.

Synthetic recommendations:
Amsoil synchromesh MTF https://www.amsoil.com/shop/by-produ...on-fluid-5w-30
RedLine lightweight shockproof https://www.redlineoil.com/product.aspx?pid=43

Conventional recommendations:
Pennzoil synchromesh http://www.pennzoil.com/en_us/produc...ion-fluid.html

I have used the Pennzoil product with good results but it's not synthetic so I only ran it for about 12,000 miles. I'm currently running the Amsoil MTF, it works great and is fully synthetic so I plan to run it for about 25,000 miles. (I don't drive hard all the time.)

As always, your mileage may vary, and some will argue with my statements. Just my $0.02 so take it for whatever it is worth.

scotth48

On another post "Oil Analysis" that 12-15,000 miles was a recommendation. Since this oil is not filtered wear from synchronizers can cause wear on other components in the transmission.

DGXR

Good point. Maybe I will service sooner than 25,000 Thank you.

GREGGPENN

Not true. The original fill was not "motor oil". It had additives specifically designed for synchros.

Castrol 10w-60 (bwm) motor oil was listed as an alternative but was not "standard fill". Several members disliked and disapproved of it's performance...especially in colder temps. As such, I don't believe it was a stellar alternative. Oil analysis after modest use wasn't stellar either.

mtwoolford

keep your GM car all GM