I got several used tin can capacitors, 40 uF @ 3 kV, about 8 pounds each,
probably film in oil. With such capacitors, is there any potential problem
if I do not keep them upright during operation??

Prune-

They will most likely operate in any orientation. On the other hand, I've had
several different oil-filled capacitors leak, so it is best to orient them with
the seals up.

73, Fred, K4DII