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Fuels & Lubricants Discussion all about Fuels & Lubricants. synthetic oil, conventional oil, brands, change intervals, diesel grades, gelling and such debated items like that. Non TDI related postings will be moved or removed. This forum is NOT for the discussion of biodiesel and other alternative fuels.

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Old July 29th, 2020, 15:37   #1
Join Date: Aug 2019
Location: Victor, Idaho
TDI(s): 2012 Audi A3
Default Transmission oil

My friend is getting his manual transmission oil changed for his 2005 Jetta TDI. Mechanic is recommending GL5 rated oil. That seems wrong! Or am I wrong?
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Old July 29th, 2020, 16:40   #2
Powder Hound
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Join Date: Oct 1999
Location: Conkud, New Hampshiyuh, USA

The story is that GL-5 supposedly is detrimental to yellow metal parts (bushings, synchronizers, possibly shims) because the extra pressure additive is molybdenum disulfide. MoS2 is great stuff as it works with pressure and does not require heat to plate the moly onto the wear surface and thus prevent wear. When this happens, the sulfur is released and can combine with the copper in the yellow metal parts. That is where the corrosion happens. Or maybe it is just a tale from 60-80 years ago, and the chemistry is better these days, but the old story won't die. Kinda like driving real easy during engine break-in.

GL-4 lubes are "better" because the amount of antiwear additive is lower. You can tell because it doesn't stink so badly as GL-5 lubes.

Some people dispute these findings. I have read anecdotal information on this site where a german engineer, when asked about this directly, said the GL-5 restriction was not necessary and their (VW's) transmissions could do fine with GL-5 lubes. Since it is anecdotal, it is merely a story and there is basically no weight behind it. But some people will claim worrying about GL-5 vs GL-4 is a waste of worry energy, and others act like you are committing sacrilege to use GL-5 in your VW transaxle.

So you can run an experiment, or you can just use only GL-4 and relax. It all really depends on how risk adverse you are, and whether you think the transaxle is already compromised or not. I guess.

If you decide to use a GL-5 lube, make sure the viscosity is down where it needs to be for the climate in which you live. Up here in New England, it is still quite cold in the winter despite global warming, so most viscosities that are available in the GL-5 category aren't appropriate anyway. So make sure that you get the right viscosity no matter which way you decide to go.

Personally, for my car, Redline MTL works great, and did so when I was in Phoenix as well. There's a little thicker version available which probably would be better for a hot climate like Phoenix, but MTL works well in either place. Lots of users here like Synchromesh (various sources, probably all made by the same Pennzoil facility), and there are some who will claim if you don't use the prescribed VW lube, you are risking (best not described here - this is a family show after all) failures. Up there in Idaho, I'd be very careful about a GL-5 lube's viscosity.
All the above mentioned lubes (MTL, Synchromesh, VW's) are described as quite thin. Please bear this in mind, or the transaxle that works so well now might sound like it is trying to chew itself to death come Thanksgiving. So.

Just think about what you (or rather, your friend) would like to do and go for it. Don't use what the mechanic wants to get rid of - use what will really work and is really recommended based on the application, not stock levels.


You always pay for what you get. Sometimes you get what you pay for.
It is called dope because it does make you dumb.
Thinking outside the box is difficult for some. They're afraid they'll fall off the edge of the box and be lost to oblivion.

Last edited by Powder Hound; July 29th, 2020 at 16:52.
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Old July 29th, 2020, 17:06   #3
Join Date: Jul 2020
Location: Brisbane, Australia
Fuel Economy: 5.0L/100km

How often should you change your transmission fluid? I don't think mine has been done in a long while or more likely never done at all (my cars at 205,000Km)
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Old July 31st, 2020, 15:26   #4
Join Date: Aug 2019
Location: Victor, Idaho
TDI(s): 2012 Audi A3

Update my buddy put in GL4 rated fluid in the transmission and now Jetta shifting 10X smoother and better the ever before. He used syncromesh fluid.
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