buy a seal swell additve think they sell them for transmissions
BUT it will/may take a few days or longer and normally need to be running to heat to swell seals.
use a thicker oil like a 20/50 to slow leak.
bruce
I dropped an old kick start shaft seal on my Yamaha in a butter dish of this stuff. Unfortunately, I forgot about it and a couple of weeks later, the seal was so swelled that the sealing lip was wrinkled.