Withoutquestion the single most important criteria for choosing lubricatingautomotive engine oil is its viscosity. Understand that the thicker anoil, the higher its viscosity. Multi-grade oils work by adding apolymer to a light-base oil. As the oil heats up, the polymers unwindinto long chemical chains, which prevent the oil from thinning too muchas it warms up.
As you already know, motor oil has a number ofbig jobs to perform, not the least of which is lubricating internalengine parts. Oil must also help cool the engine by transferring heatand by sealing the piston rings and cylinder bore interface. It mustalso absorb contaminants, and suspend wear particles and soot thatforms as a result of combustion.
Looking deeper, part of oil’sjob is to stop the metal surfaces in the engine from grinding togetherand tearing themselves apart from friction while transferring heat awayfrom the combustion cycle. It must also be able to hold unwantedbyproducts of combustion, such as silica oxide and acids in suspension– at least until they can be safely removed by the filter.
Oilmust clean the engine from chemicals and other build-ups, keep themoving parts coated, plus minimize the exposure to rust-causing oxygenwhile holding down higher engine temperatures.
Here’s anotherimportant consideration for your shop. How long should oil be kept onthe shelf? In general, liquid lubricants (oils and greases) will remainintact for a number of years; as long as four or five. However, asyears pass, unused engine oils can become obsolete and fail to meet thetechnical requirements of more current engines. In other words, stay upto date on newly introduced engines and the type of engine oiltechnology they require.
In the meantime, more and more oilcompanies are coming out with many “high mileage” oils. Some arerecommended for engines with as many as 75,000 miles on the odometer.Generally speaking, these high mileage oils have special additives inthem, so they are more suited to older engines. The first is normally aburn-off inhibitor that helps prevent the oil from burning off as itgets past an engine seal and into the combustion chamber.
Anotheris a “seal conditioner,” designed to soak into seals such as headrocker-cover gaskets and force them to expand. If one of the seals is abit leaky, the seal conditioner will attempt to minimize the leak.