A friend's Model A has a '39 V-8 trans, he FILLED it with STP! It was TOO slippery for the synchro braking rings & totally foiled the synchro action & it shifted like a stock Model A. Took him forever, with the trans top off, to WASH all that CRAP out using solvent/gasoline/and even lacquer thinner, then it took even more driving for the synchros to SLOWLY begin to do their job again. I HOPE NO ONE EVEN THINKS ABOUT USING STRAIGHT STP!!! It ain't a gear oil, it's just some sort of slippery booster??? My thought is, as an additive to motor oil, it CLINGS well & helps on a dry bearing situation on start up. Don't put too much, as Model A's motors are primarily GRAVITY lubricated. I've used it at half recommended amounts with multigrade oils. With straight30 weight, I think it would be too thick to flow well! A fair rate of flow is necessary to keep bearings cool.(But what do I know?)
If I live long enough, I will build an oil cooler pan for my "A" engine. A friend, in the '50's built one for his Flathead V8 powered "Hardtop" oval track car. Drilled 5 or 6 holes in the front & back of the lower extremity of the pan to accept lengths of 1/2" copper tubing with the front of each flared out quite large. Then he had a radiator shop guy silver solder them in! He had an engine oil temp guage & it did make quite a difference in oil temperature. Bill W. (The Dog wasn't around then, so he can't back me up on this!)