could be leaking a bit of gear
lube from the lower unit. If you had a motor oil leak
from the power head
it would be visible on the block and around the base inside the cowling. Also over time you would notice an oil
loss with the dip stick.
lube looks just like motor oil when it breaks down, especially in salt water
. Check the tightness of the filler plug
and "o" ring on the lower unit...also check leaking when it is out of the water
on a bracket or mount.