Dang it man, I bet you had to dig deep to find this year old topic so you could plug an ad for penzoil.....lol J/K
The switch seemed to work for you that's good
I thought the 15W 40 castrol was a sythetic though?
Just thinking out loud here - but you might have gotten similar results going to a 20W 50 weight oil.
You went up 10 on both ends of the oil going with that 25W 50 veres the 15-40 - just a observation>Cp
Cp