First off- make sure it's primary and not gearbox oil.
G/B oil has a pungent odor and will leak down behind the primary cover.
Try and find a surface plate or piece of glass to check for flatness of the cover and file out the high spots.
Just had the same problem with mine and am using 1207b Suzuki form in place gasket sealer.
Had to remove the alignment dowells in 2 places to help it break the case free in the future (hopefully).
Even made my own gasket last time but they never seem to survive being removed without damage so am trying what I had in lieu of.
My neighbor has a cast iron surface plate so we used that to find two major high spots due to over-torqueing.
Mine is still a problem with the chain adjuster access screw and o-ring, but this is behind the cover itself.
As yours is a bit older, not sure how the drain plug would affect leaks.
Someone on another forum found a piece of polished granite scrap big enough to use as a surface plate- broken scrap from a stone shop or big-box home center.