Not clear what spraying the exterior of either drain or fill plugs might accomplish ...Lots of good advice here...btw...get a long 10mm hex wrench and you'll have enough leverage. Also a tap hammer, tap 15 times lightly on the bolt...also spray it with PB Blast and let it set for 30 minutes...it's a fun job to change all the fluids...
a) The copper washer seal is outboard of the threads, so there is no way that any externally-applied lubricant will get past the seal and do anything useful.
b) Being submerged in gear oil for years, the threads are already well lubricated, and it's only the tension in the threads of the tightly-torqued plugs that makes the plugs so difficult to remove, at least the first time.
Rapping sharply on the head of the plugs multiple times with a medium-sized hammer is the only way I've found to ease removal of some of these plugs (first removal after original factory installation).