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Why Switch Motor Oil Viscosity in Different Seasons
 
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Question:
Why Switch Motor Oil Viscosity in Different Seasons
My '99 Grand Prix has the 3800 engine which calls for two different viscosity engine oils. 5W-30 when temperatures are below 32° and 10W-30 when temperatures are constantly above 32°. My question to you is the following: Since Mobil 1 5W-30 and Mobil 1 10W-30 have approximately the same cold pour temperatures, would it be okay to use 10W-30 year round? Coldest expected temperature is minus 20°.
-- Jim Dalrymple, Horseheads, NY
Answer:
The key difference in these two viscosity grades of Mobil 1 does not necessarily lie in the Pour Point measurement, but with the results of an industry low-temperature viscosity test called the Mini-Rotary Viscometer (MRV). This test measures the ease of which an oil is able to be pumped around an engine at low temperatures. Mobil 1 10W-30 can certainly be used adequately in that temperature range, but we would recommend following your owner’s manual for viscosity guidance. By switching to a 5W-30 in colder seasons, when starting your car on a cold morning you are providing your engine with an oil which can be pumped more easily to the moving parts of your engine (5W-30 having a lower MRV viscosity than a 10W-30 at equivalent temperatures). Your owner's manual is calling for a 10W-30 engine oil in warmer seasons to ensure your engine is protected at elevated temperatures by providing a thicker oil film for protection.
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