Quote:
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Originally Posted by Brucelee
I am thinking Mobil 1 full syn gear oil 75W-90.
Jim?
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Hi,
You certainly could use Mobil 1, but the benefits probably don't outweigh the increased cost, and since it's rated GL-5, it
may not be the best thing to use.
This is because Hypoid Gear Oil is used mainly for the Differential which uses Hypoid Cut Gears. Because of the sliding contact that Hypoid gears make, (
Hypoid gear teeth appear twisted) , their Hydrodynamic Contact pressure is higher. To be suitable for use with Hypoid gears, a lubricant must be capable of resisting these higher pressures and is rated as
EP.
A Manual Transmission won't usually contain Hypoid gears, so it
doesn't need an
EP oil. But, in Transaxles (as in the Boxster) where the
Crownwheel and Gearbox share the same lubricant, Hypoid Gear Oil is needed.
Hypoid Gear Oils contain greater amounts of Sulphur or Sulphur/Phosphate compounds to withstand the higher Hydrodynamic Pressures and prevent
Shearing. They have a typical
Rotten Egg smell to them, especially after some service time.
One problem with these compounds is that they can chemically react with the parts in the Transmission, especially
Yellow Metal Parts such as some Synchros and Bearings. I cannot find any information on the composition of the Tranny's internals, but if they are using any Brass, GL-5 Lubes can cause premature wear. This is why I would not use the Mobil 1. Instead, I'd go with a GL-4 Rated Hypoid Gear Oil - same Lubrication and Compressability qualities, but does not attack
Yellow Metal Parts.
Mobil 1 touts their benefits as withstanding Higher Temps (not an issue here), Longevity (again, the Tranny is a sealed system and not subject to Dirt, Moisture which shorten the Lubricants Lifespan) and lower Viscosity (again, not an issue unless you're in Alaska, especially if you drive passively until the Gearbox has warmed up - which you should).
Personally, I just don't see the need and the fact that there
could be some Brass in the Tranny causes some concern. It seems to me that possible accelerated wear to the Tranny and higher cost are real negatives. There are lots of GL-4 rated Lubes out there which will work just fine - I'm not sure there is a
better in this case.
But, since the Tranny is sealed and unfiltered, it is prudent to change the Gear Oil every 3rd year to remove any of the metal shavings which will accumulate. Not more often because Gear Oil is actually better if a little aged - the Fresh Additives will not be especially good for it. Hope this helps...
Happy Motoring!... Jim'99