Question about motor start and run capacitors

[Martin Whybrow]

I need to source some capacitors for a rotary phase converter I'm building;
my usual supplier lists polypropylene film motor run capacitors (by LCR) at
a good price, but I need a large quantity of motor start capacitors as well.
Anyone know if motor run caps can be used for motor start duty? I know that
using motor start electrolytics for motor run applications leads to
explosive failure and I assume film caps are not normally used as start caps
because of the price.
TIA, Martin