Automotive companies universally specify multi-grades in owner’s manuals and SAE 30, which was the preferred oil for GM’s 7.4-liter engine in the company’s motorhome chassis for many years, is no longer listed as suitable.
Owner’s manuals for Ford motorhome chassis and light trucks, for example, list 5W-30 as preferred for gasoline engines in all temperatures and 10W-30 as suitable for temperatures above 0? F. GM light-truck manuals specify SAE 5W-30 as preferred and 10-W-30 as acceptable for temperatures above zero. GM recommends against using any other grades of oil, specifically 20W-50.
Engineers suggest that the viscosity spread 20W-50 is too great, requiring too many additives. Many years ago GM advised against use of SAE 10W-40 for the same reason, although other motor companies continued to approve that grade.Federal fuel-economy regulations also motivate manufacturers to recommend relatively low-viscosity multi-grades. Also, there is the possibility that the vehicle owner who sees heavier oil may neglect to change to a lighter oil for cold-weather driving. Single-grade oils create additional resistance to movements of engine components under all conditions, compared to multi-grades with lower reference numbers. These companies apparently do not believe most motorists are conscientious enough to avoid misusing single-grade oils.
In support of the recommendations, engineers admit that they are motivated by fuel-economy considerations in vehicles with a gross vehicle weight rating (gvwr) of less than 8,500 pounds, but add that they are convinced the latest lightweight multi-grades carrying the starburst symbol will protect engines under extreme operating situations. One veteran engine-design manager admitted he had always favored thicker oils for hot-weather situations but was won over by his own engine durability testing with the lower viscosity oils.
This Article provided by The RV Handbook.