How many readers, when asked which oil they’d like put in their car at their local automotive service centre, have looked at their serviceman in response, with an abject loss for words. The correct answer is not one we’re all inclined to memorize, and one may hazard to guess that “whatever you think is best” is regularly cast as the dominant reply. Those numbers on the side of that bottle of slippery goo do indicate facts about its content. Here’s an overview to explain what they mean.
For starters, the obvious basics: what’s this stuff for in the first place? Contacting metal parts moving against each other at high speed are detrimental to the long-life of your internal combustion engine. Oil provides a lubricating film to minimize wear caused by friction between such adjacent moving parts. This friction, if not kept to a minimum, will rob your engine of useful power, increase your fuel consumption, decrease the life of your engine’s internals and, as a worst-case scenario, will ruthlessly blow your engine apart if run dry of any lubricating product.
Forming a seal between pistons and cylinder walls to prevent the leakage of combustion chamber gas, is an additional requisite of any oil. Another is cooling, as is its job for the pick-up and removal of sludge and metallic particulate. Corrosion and rust inhibition are the final major benefits of its presence. Oils are primarily petroleum based (these are referred to as mineral oils) though synthetics exist that are made from artificially synthesized polymers.
The subject of oil cannot pass without reference to the topic of viscosity. Simply put, viscosity describes the thickness of the oil to which it relates: thicker better maintains a proper lubricating film though has greater resistance to flow, while thinner flows better around parts to ensure they remain constantly well-coated.
Oils are coded using a baseline system devised by The Society of Automotive Engineers, SAE. Referenced at a temperature of 100!C, oils are graded with SAE numbers – 0, 5, 10, 20, 30, 40, 50, 60, or 70 – that relate to their viscosity levels and inherent ability to flow at the referenced temperature. Thus, the higher an oil’s viscosity, (and, therefore, its resistance to flow), the higher the SAE number, or grade, applied to describe it. The “weight” of an oil is a commonly used term referenced back to these numerical values.
Oils behave differently depending on prevailing ambient temperatures. When cold, oil can become like syrup, so keeping it thin to perform its tasks is a crucial requirement of its blend. Conversely, searing conditions can make oil about as useful as water, so keeping it thick will maintain its constituent properties.
To compensate for ranges in temperatures, polymer additives are conjoined to single-grade oils thereby making them multi-grade types. The viscosity of multi-grades is much more consistent across ranges of temperatures than are oils of single-grade varieties. Multi-grades are what you’ll put into your car and aircraft; for the latter, your POH may typically designate 15W-50 as a predominantly recommended type.
The “15W” – where the “W” means that the oil’s viscosity testing has been undertaken in winter conditions – indicates that your engine can pump this oil through itself as well as it can a single-grade 15 weight oil, irrespective of temperature. The “50" designation means that the viscosity of this multi-grade oil at 100!C corresponds to the viscosity of a single-grade 50 oil performing at the same given temperature.
Commonly used in general aviation is oil referred to as ashless dispersant. While straight mineral oil is generally used for breaking-in an engine, “ashless” means that the product is free of metallic components thereby reducing residual deposits of metallic ash that can otherwise damage an engine. “Dispersant” means that the oil will hold harmful small particles in suspension, allowing them to be carried away and filtered out by the engine.