manual tranny oil

[Rob Clark <rclark@xxxxxxxxxx>]

This last weekend I changed the tranny oil in my E30, and I 
realized that I don't really know much about transmission oil.
According to the Bentley, the normal tranny oil for this car 
is SAE 80 GL-4 non-hypoid.  The Bentley mentioned that one 
could use straight weight engine oil in colder climates, but 
not multiweight.  Why?  Does the no-multiweight apply to 
heavier weight gear oil?  Also, what is the difference between 
"hypoid" and "non-hypoid" oil?  (hypoid has to do with gears 
whose axis cross at right angles... ie transaxle, but why would 
the oil be different?)  Also, what does the GL-4 mean?

I have seen some technical info about motor oil on the web, but 
haven't stumbled across any gear oil info.  (Other than that one 
should put redline MTL in their transmission.)

- --Rob
87 325is

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