Years ago, an import shop could consider its shelves well-stocked if it contained a selection of single-viscosity oils in detergent or non-detergent blends. Today, a shop owner is confronted with stocking a variety of multi-viscosity oils, including 0w-20, 0w-40, 5w-20, 5w-30, 10w-30, 10w-40, 15w-40 and 20w-50, not to mention stocking some of these viscosities in synthetic and non-synthetic bases.
Although the number of oil viscosities currently available is bewildering, it’s also important to remember that the most important factor in selecting motor oil for specific applications is the quality of the oil’s base stock and its particular blend of additives.
With that in mind, it’s also important to remember that motor oil must efficiently perform at least six different jobs in the engine’s crankcase. It must, for example:
1) reduce friction between moving parts;
2) seal piston rings against the cylinder wall;
3) cool pistons and engine bearings;
4) cushion bearings against shock loading;
5) clean sludge and varnish from engine parts; and
6) prevent corrosion while an engine is parked or in storage.
In addition, application-specific oils might, for example, be formulated to prevent piston rings from sticking or the camshaft from scuffing in particular applications.
MODERN ENGINE LUBRICATION
Before we begin to understand the need for so many different engine oils, it’s necessary to understand the lubrication requirements of modern engine design. The primary difference between a modern engine and one of two or three decades ago are greatly reduced oil clearances in bearings, pistons and piston rings, more precise machining and higher power output per liter of displacement.
Because modern engines depend upon three-way catalytic converters to reduce exhaust emissions, it’s particularly important to prevent oil, oil ash and oil additives from entering the exhaust stream and contaminating the converter.
To reduce oil consumption, engine manufacturers have greatly reduced piston, crankshaft and valve guide clearances. They’ve done this, for example, by using hypereutectic materials in some piston designs; precision-boring and honing of cylinder walls; micro-polishing of crankshaft journals; and using more durable materials in valve guides and valve guide seals.
In all cases, oil viscosities have been reduced to flow through tighter engine clearances during start-up and cold-engine operation. Other, less visible issues also affect how oil is formulated. Engines with hydraulic lifters, hydraulic timing chain tensioners, and variable camshaft timing, for example, require anti-foaming agents to prevent the oil from retaining air bubbles that reduce the efficiency of these mechanisms.
Because overall volumetric efficiency has been vastly improved during the past several decades, engines are designed to operate at crankshaft speeds and bearing loads unheard of a decade ago. Unfortunately, many of the additives used a decade ago to increase the load-bearing, anti-scuff and anti-shearing capabilities of conventional motor oils have been reduced or eliminated from oil formulations to prevent damage to the catalytic converter.
Modern overhead-camshaft engines don’t, however, require as much anti-scuff and anti-shear qualities in motor oil to reduce camshaft wear as do older pushrod engines.
In addition, the oil change intervals of modern engines have doubled or tripled in length, so modern motor oil is required to have a longer-lasting additive package than ever before. Without adequate additives, conventional oils tend to oxidize and lose their detergency long before the oil change interval expires. The result is the oil gelling from oxidation or a heavy coating of sludge being deposited in the valve train and crankcase areas.