Trouble finding GL-4 gear oil? (GL-5 is hard on bronze)
From the folks at Castrol:
Our Hypoy gear oil line has always
been based on the military specification
MIL-L-2104 and API GL-4. Our Hypoy C gear oil line is based
MIL-L-2105/API GL-5 specifications. As these specifications
so were our products.
Hypoy 90 was an API GL-4 product. If you need something
your best choice out of our line would be Hypoy 80W-90. Although
it is a
multigrade, it still provides the high temperature viscosity
of Hypoy 90.
I'm personally a great fan of Redline synthetic oils. Redline
MTL works great in a TriumphTR4 overdrive gearbox
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