Sounds normal. There is a passageway in the tube of the oil filter cap that could be blocked or restricted. I'd clean mine periodically with brake cleaner and compressed air, wear eye protection IT'S MESSY.

Didn't know the blend is 15W-40, what I had in mind was 5W-40 full? synthetic, was ages ago.

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer