Used GL-5 oil in GL-4 transmission, need advice...

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Oct 19, 2004
Hello all,

I have been using Mobil 1 synthetic gear oil in my VW tranny for about 75K miles. I recently changed it because it was getting very difficult to shift gears. I put in the same oil and it shifts pretty good now, although it could be better.

I addressed this issue with some people and found out that this oil is infact corrosive to my transmission's internals. Since I have been using it for so long is my transmission likely to break soon? I'm going to put Royal Purple Max Gear in there as soon as I can, does that sound good? Thanks for any feedback.

Skoda Octavia 1.8T 2000, 135K KM.
I'd go with Redline MT-90 or Specialty Formulations MTL-R. Both of these are safe for VW trannys. The Royal Purple gear oils are somewhat thick especially at 40C, so I would not use.

The older GL-5 formulas did attack brass synchros, but today the newer formulas are somewhat safer. There has been debate whether Mobil GL-5 gear oils are safe for GL-4 applications, but I personally wouldn't use it.
Bobert, what are the advantages/disadvantages of taking one of the Redlines over the other? Thanks for your input.
Redline makes two GL4 products: MT-90 (75w-90) and MTL (70w-80). Link to Redline . VW's mostly use 75w-90, unless you have one of the newer models which use a thin viscosity gear lube. What kind of VW and tranny do you have?

Specialty Formulations is another company which makes MTL-R (75w-90). It's dual rated GL-4/GL-5. Link to Specialty Formulations
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I have a 2000 Skoda Octavia(VAG). It uses a 5 speed VW transmission. Thanks for the links, the Specialty Formulations MTL-R seems interesting. Which one do you think is better, Redline MT-90 or SF MTL-R?
I've tried both the RL MT-90 and SF MTL-R in my Tacoma (2001) trany and I prefer the SF. The RL just seemed 'notchy' to me.
Ok didn't realize Skoda uses a VW tranny.
I think either gear lube is very good, but MTL-R gets my vote.
A flush is probably unnecessary. By the time you drain and refill, any corrosives will be diluted to the point of becoming harmless.

If you're worried about it, just run the new oil on a relatively short OCI, say about 1 year or 12,000 miles.
Is there anything I should do to clean out the GL-5 stuff first? Or run something that can "recondition" the gears, before putting in the new oil?
I wouldn't waste time flushing, just drain and go. Besides you already replaced with M1 so this should act as a flush if you drain and replace again. Don't worry about the few drops of GL-5, it will be so diluted it won't matter.
I don't see sulfur as the problem. In order for acid to form, there has to be 'acid forming conditions'. Running the fluid for an extended time, running low on fluid or too hot, or letting it get full of moisture, IS a problem. Anything, that breaks down the additive package and buffering agents, is what you should be worrying about.

So, don't worry about the GL5 gear oil.
Do practice a safe change interval every 2-4 years, or every 30k miles, or when you pull the vehicle out of a big puddle(floodzone or offroaders understand this).

I've used Mobil1 gear oil in manual trannies and never experienced syncrho issues. M1 also shifted better then the OEM fluids. But, the boutique manual fluids do shift better. And, if the transmission family has any type of negative history concerning gear, shaft, fork, linkage, or bearing wear/breakage/issues, I'd feel better with the GL5 level of protection.

I had the GL5 in there for 75000 miles, under really strenuous conditions at times.


I agree, this stuff should act as a nice quality flush.
That M1 is good, but is more of a rear end oil, and not well suited to manual transmissions. It will have great protection, but shifting is often marginal - it is too slick and not friction modified for synchros.
M1 tech support specifically does not recomment this sauce for manual transmissions.
Castol in Europe make a range of GL-4 MT fluids that work very well. Some are rated for certain Opel and BMW models, as well as ZF 'boxes used in Mercedes medium and large truck synchro gearboxes and Volvo large truck transmissions.
I've used Syntrans 75W-85 (12 cSt @ 100*C, Volvo and ZF approved) with great success in Land Rover R380 gearboxes.
Yeah, I'm in Germany. I didn't even occur to me to look at the "local" market given the usually high prices. I already ordered Redline MT-90. I'll try one of those Liqui-Molys if I don't like the Redline's performance.

When I shift I have to wait half a second for the gear to engage before completing the throw. I hope I'll be in shifting nirvana after putting in a GL-4 oil.


What exactly does 12 cST @ 100C designate? The lower the cST the better?

Thanks for all the replies.
I just put in RL MT-90 a week ago and the shifting is tenfold improved. Thanks for the recommendations. Initially though, the first few days, it was very difficult to shift, then one cold day it was possible to shift with relative ease. I guess the fluid had to break in or something.
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