I posted some of what I have learned about GL4 - GL5 oils on a VW site that I reference for a beetle that is being restored. What I thought may have been true at one time, but maybe not now. Following is an email (to someone else) from an Amsoil tech regarding 4 vs 5. Or you can check the whole thread here http://www.thesamba.com/vw/forum/viewtopic.php?t=276529
Here's an explanation Bruce sent me almost 2 years ago when I had the same question. Figured I'd help you not to repeat yourself again, great info!
Jeff, All cars made today have brass syncros that look the same as Beetle syncros. To think that the oil companies would make a product that would destroy every gearbox it went into is rediculous. When I first heard this GL-4/GL-5 corrosion thing, I did some research on the oil co web sites. I couldn't find anywhere that said it was harmful, so I sent a tech message to the Amsoil tech guy. His response stated the history of how that myth got started by the ignorant VW people. When GL-5 first came out it was indeed corrosive to brass syncros. But only at temps above 250ºF. Now since it is impossible to get any street driven gear oil to that temp (It is hard to get engine oil that hot!), there will be no problems using regular GL-5. However, just to satisfy the ignorant masses, the API came up with another added rating, called MT-1 which is tacked onto the end of all GL-5 oils you can buy today. With this MT-1, GL-5 oil is not corrosive at any temp.
The difference between GL-4 and GL-5 is the extreme pressure capability. On surfaces that have high pressure where metal to metal contact will happen, the GL-5 is way better at protecting. Spider gear teeth and side gear teeth are one place where GL-5's better EP rating is desperatly needed. I've got 5-6 Bus 091 ZFs in my garage right now that have suffered significant wear on the gear teeth from probably using GL-4.
The nay-sayers like to point out their Bentley recommends GL-4. Of course it does, that's all there was 30 years ago. It also recommends using only mono-grade engine oil, bias ply tires........
The use of synthetic or dino makes no difference. I highly recommend you use syn over dino, you can really feel the difference in the way it shifts. If you buy a fresh gearbox, first put in dino for about 500-1000 miles then dump it and put in syn. It stays there long enough to warrant the extra cost, unlike syn engine oil.
I put my money where my mouth is. My last gearbox was a full out Berg 5 with close 2nd and a near NOS ZF. You couldn't pay me enough to risk damaging it with GL-4's inferior EP performance. Argueably, that gearbox is worth $3500-4k.
I always challenge the guys that say GL-5 is bad by asking them to tell me who do they know that had a failure using GL-5. Still waiting.
Good luck,
Bruce
_________________
So, if this is true, some current GL5 oils are ok for the mog trans. Just thought I would share.
Here's an explanation Bruce sent me almost 2 years ago when I had the same question. Figured I'd help you not to repeat yourself again, great info!
Jeff, All cars made today have brass syncros that look the same as Beetle syncros. To think that the oil companies would make a product that would destroy every gearbox it went into is rediculous. When I first heard this GL-4/GL-5 corrosion thing, I did some research on the oil co web sites. I couldn't find anywhere that said it was harmful, so I sent a tech message to the Amsoil tech guy. His response stated the history of how that myth got started by the ignorant VW people. When GL-5 first came out it was indeed corrosive to brass syncros. But only at temps above 250ºF. Now since it is impossible to get any street driven gear oil to that temp (It is hard to get engine oil that hot!), there will be no problems using regular GL-5. However, just to satisfy the ignorant masses, the API came up with another added rating, called MT-1 which is tacked onto the end of all GL-5 oils you can buy today. With this MT-1, GL-5 oil is not corrosive at any temp.
The difference between GL-4 and GL-5 is the extreme pressure capability. On surfaces that have high pressure where metal to metal contact will happen, the GL-5 is way better at protecting. Spider gear teeth and side gear teeth are one place where GL-5's better EP rating is desperatly needed. I've got 5-6 Bus 091 ZFs in my garage right now that have suffered significant wear on the gear teeth from probably using GL-4.
The nay-sayers like to point out their Bentley recommends GL-4. Of course it does, that's all there was 30 years ago. It also recommends using only mono-grade engine oil, bias ply tires........
The use of synthetic or dino makes no difference. I highly recommend you use syn over dino, you can really feel the difference in the way it shifts. If you buy a fresh gearbox, first put in dino for about 500-1000 miles then dump it and put in syn. It stays there long enough to warrant the extra cost, unlike syn engine oil.
I put my money where my mouth is. My last gearbox was a full out Berg 5 with close 2nd and a near NOS ZF. You couldn't pay me enough to risk damaging it with GL-4's inferior EP performance. Argueably, that gearbox is worth $3500-4k.
I always challenge the guys that say GL-5 is bad by asking them to tell me who do they know that had a failure using GL-5. Still waiting.
Good luck,
Bruce
_________________
So, if this is true, some current GL5 oils are ok for the mog trans. Just thought I would share.