1971 VW Beetle

[Marc]

The current GL-5 lubricants which meet the MT-1 spec are definitely better than any old stock which does not, but you can do better.

Never use any lube, synthetic or not, that doesn't meet the The American Gear Manufacturers Association (AGMA) rating D-130. Differentials (like the final drive portion of an automatic transmission or the third member of a conventional drivetrain) do not contain synchro rings made of non-ferrous materials so they can use lubricants with a higher amount of extreme-pressure additives; problem is that when these additives break down into sulfurics which corrode the brass in the old transmissions' synchros. Generally this means you need to avoid GL-5 lubes and look for GL-4, but it's getting harder and harder to find. API GL-5/MT-1 is supposedly safe to use in old transmissions if GL-4 is unavailable - GL-4 is still produced, but not too many places bother to stock it anymore. CRC/Stalube still offers GL-4; NAPA and CarQuest have a GL-3/4/5 which should be fine too.

ASTM D-130 is normally run at 212ºF, but to make the test more severe AGMA increased the temperature to 250ºF. ASTM D-130 is used to determine the corrosiveness of a gear oil’s EP additive to nonferrous metals. A polished copper strip is immersed in the 250ºF oil bath for three hours and the EP additive corrosiveness is determined by comparing the strip against ASTM pre-rated strips.
ASTM rates the test strip as: 1 a or b; 2 a, b, c, d or e; 3 a or b; or 4 a, b, c or d.
Ratings of 1 a or 1 b are the least corrosive.

...If I was putting in a synthetic, it'd be Redline MT-90 GL-4 or Royal Purple Max-Gear 80w90 GL-4/GL-5. Royal Purple meets the GL-5 spec but their "Synslide" EP additive is non-corrosive so it's no threat to the VW synchros. Another choice would be Amsoil, I haven't used it myself but it's claimed to be safe for GL-4 applications...

[Jim Ed]

I would prefer to keep using synthetic GL4 80W-90 as long as I can still get it.

[mountainkowboy]

Finally got the broken bolt out, its a good thing I started spraying it with rust dissolver yesterday. Replaced all the bushings and put the single spring plate on along with the trailing arm. I still need to pull the brakes apart and replace everything back there. Parts are ordered, but are not here yet. New park brake cables and steel braid rear brake hoses (black Friday sale). I also need to get to Socal Imports to pickup the rear drums in the right bolt pattern that I need. There has been a hic-up in the trans department, so I guess that I will pull the one out of the Cabrio and take it into KCR to be rebuilt. In the mean time I will get the front up in the air and tear it apart.