Mine came with a small bottle of testing solution, I think it is the same stuff use for medical sonic testing scope
I have use white lubriplate type grease and got the same numbers so maybe you can to twocents
Find a smooth thin machine area on a head or block and measure it with a micrometer or even a set of dial calipers and then measure it with your sonic tester and see how much difference you get scope If it is a small amount, less than .0030 I would use it and go from there twocents
Can you calibrate your tester to a known thickness by adjusting the frequency ? If so maybe you can do that also scope work

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)