2010 Silverado 1500 with 140,000. I switched from Valvoline High Mileage synthetic blend to Valvoline High Mileage full synthetic both 5w-30 and from a Valvoline VO88 oil filter to a Wix Xp. Why did my oil pressure go from 41-42 psi at cold idle to around 28 at cold idle after switching?
