I ran his 680 in my car for about 300 miles this past summer and fall - nary a squeak. However, I'm injecting the oil into the steam line right where it enters the valve chest, and the temperature is probably lower there than right out of the superheater in the steam loop. Maybe it wouldn't be a useful comparison.
He's been messing with steam cylinder oil for a long time, although mostly selling to locomotives.
Kelly
P.S. If you put a tee on the steam line into the valve chest, so that you can inject the oil there, don't mount the flare joint at a 90 degree angle to the engine's axis of torque rotation... makes for a big steam leak after about 200 miles.