Capacitor voltage and corona

At what voltage do you have to start submerging capacitors in oil to reduce
corona / improve insulation?  It would appear that a lot of the bother with
rolled polyethylene caps is the juggling of several layers of dielectric and
the mess / bother / weight of submerging them.  Is there any merit to
combining some of the MMC approaches with a group of dry, single 6 mil poly
/ Al foil caps in series?  Judging by the prices of MMC caps, this isn't
going to be a cost saving issue as much as availability for some.  I live in
a <350,000 pop center with no suppliers of any of the types of caps being
referenced, and living in Canada makes mail order subject to nasty surprises
with unpredictable charges from customs and extreme difficulty in returning
incorrect / warranty orders.

Brian Howden