recommended oil for well tempered spindle cup

Since the spindle and bearing of your WT TT are unlike any others, why not follow the advice from Firebaugh? The truth is that almost any good lubricant without additives will work.

So Bill, are you saying NOT to use a single weight, low viscosity, nondetergent, synthetic motor oil, which could also be called an "engine oil"?  Sounds like you used more ordinary engine oil (likely to be multiviscosity and with additives including detergent) to coat your tools when you were a lad. And in any case, none of us is keeping his TT in an unheated garage over the winter.

I am sure your direct experience is more cogent than my guesswork, but one might think that with a stretchy belt, higher viscosity would engender more belt stretching which might lead to more speed irregularity.  Just thinking out loud. Anyway, my bottom line contention is it doesn't make much difference what lubricant you use.  If I recall my friend's WTRP, the "bearing" actually consisted of some white protrusions in the well, maybe one or two rounded hemispherical white plastic mounds, which contacted the spindle when the belt pulled the platter toward the motor.  Very unconventional to begin with.

There is just no need for two weights, unless you’re planning on 5000 rpm.

This is not to contradict Firebaugh. He was probably exaggerating. Or picked some numbers out of the air. By all means do go with his recommendation.

33 or 45 rpm is not much of a stress on any bearing using any oil. The Mobil One recommendation is good as any.

What source could be better than “Mr Firebaugh”?

But I use single weight, synthetic, nondetergent motor oil for all my TTs. I have never owned a WT TT.