Q: How important is viscosity rating in current engines? I have been using Castrol full synthetic in all my vehicles for the last 10 years, and almost all recommended 5W-30. I have a 2008 Ram 1500 with a 3.7-liter V-6 that recommends 5W-20 to the point of putting it on the dipstick. Last week, I bought Castrol Edge and didn't realize I had gotten 5W-30 until I was pouring it in the engine. I checked the manual and found 5W-20 is recommended for all Ram engines except the 5.7-liter engine operating under a gross combined weight rating of 14,000 pounds. Will the 5W-30 harm my engine, or is the 5W-20 just to improve fuel mileage?

A: Manufacturer recommended oil viscosities are steadily thinning out. Over the past few years many have taken the next step to 0W-20 engine oil. The drive is to meet Corporate Average Fuel Economy regs. The advantage to the lower viscosities (thickness under hot and cold temperatures) is to reduce friction (drag) on engine rotation, meaning better cold engine startup and improved fuel economy under hot running conditions. The advance in engine oil formulation, along with presumed technology in engine design and machining, makes this possible without causing premature engine wear. Until recently, manufacturers included an engine oil viscosity chart in the owner's manual providing a choice in viscosity depending on current ambient temperatures. But now most are sticking with a specific multi-grade oil recommendation for all temperatures. To answer your question, no, 5W-30 instead of 5W-20 will not harm your engine, even in cold temperatures. But if you're still worried, by all means repeat the drain-and-fill.


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