Two steps forward and a giant leap backwards.

Good that you checked the oil.
Though oils are different now...just remember the old standard..
"Change every 3 months or 3,000 miles"

Only if the car is in a climate controlled storage can you ignore that. "Heated"

If you use good oil you can double the milage IF you are driving it all the time. ..that's that "heat" thing again...

That aside check the intake manifold for a leak at the cross overs