This is a shot in the dark, but I remember reading that M112's of this vintage have a problem with the breather system getting fouled that can cause this.

If a dealer did all the service that engine should have been on Mobil 1 0w40 since day one, now whatever 5w40 they switched to. Its just gotten enough miles on it to be broken in.

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2016 Corvette Stingray 2LT
1969 280SE
2023 Ram 1500
2007 Tiara 3200