There are seals on each side of each of the crank lobes, so thats 6 seals that stop anything leaking between cylinders. Over time they get brittle and start cracking, if air leaks in the cylinder runs lean and very bad thing happen.... like seizures and holed pistons.
The seals do not sit in oil so having oil in the engine is not a cure.
A leakdown test will give you an idea of their condition but personally unless I changed them in the recent past or was positive someone else did I would (and did) take it apart and see what shape they are in. You will save money and heart ache.