Thanks to everyone who responded.
Before I start collecting old shirts and underwear to soak up the mess, I 
thought I'd speak to Mobil, from whom I purchased the synthetic 75/90 gear oil. 
(It seemed such a good idea at the time.)
A very nice lady named Lucille told me to get it out of there and go to GL4. 
She named a Castrol product. The problem, apparently, is a sulphur/phospherous 
additive that attacks yellow metals. She told me the problem occurs largely 
at temperatures above 150 to 160, (which is probably above 30 mph!)
Thought I'd share that.
Mike N
 
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