I had the same symptoms before my rear main engine seal failed.

You have excessive crankase pressure. This is caused by a plugged flame trap, a plugged oil vapor separator (oil trap) (located under the manifold) or a plugged vacuum line from the flame trap housing to the vacuum tree (near the throttle. The flame trap is easy to clean or replace. The oil trap costs about $25 however the labor is expensive due to a requirement to remove the engine manifold to get at it. Suggest that you examine the small plastic line and two connectors which go from the flame trap housing to the vacuum tree. After several years of being ignored it clogs solid with oil residue that turns into a solid black carbonized mass and prevents vacuum from being applied to the positive crankase ventilation-flame trap system. The result is excessive crankase pressure which has several symptoms--leaky oil filler cap, excess pressure and smoke out of the dipstick holder when the dipstick is removed and the engine is running. Eventually, a rear main engine seal fails due to the excessive pressure and your spend $500-800 to get it fixed.

It cost me $4 for the 6 inch plastic line and two rubber connectors I needed to replace the vacuum line. No more filler cap leak. No more excess pressure from the oil dipstick line. Hopefully, I won't have to replace rear main seal again either.

Dan