I think RR's nailed it there. Drain the primary drive, apply half a dozen rounds of plumber's teflon tape to the threads of the drain plug and reinsert. I've been doing this each oil change on my car for the last 15 years for the same issue, works for me. How's life in Guam these days?
Nb. Refill primary before setting off again. But I'm sure you don't need me to tell you that...