Perhaps I should chirp in here. My 4 MOT's are under oil and I think
I gave them an initial load to heat and vibrate to allow the oil to
spread a bit better.
I also have under oil two paralleled microwave oven mains filters. I
always run with a Terry filter as well, so I have the output
protection. If I recall the output with resonant rise is some 11 kV for
the 4 when
hooked up to the tank cap alone.
It has been rock solid in this application run by a 15A variac without
ballast and currents which I have never really measured well but
typically over 20A.
I guess the Terry filter is the difference.
Peter
http://tesladownunder.com/tesla_coil_4inch.htm#PowerSupply