The pour point of an oil is the lowest temperature at which it can be poured. The viscosity of oils rises as they cool and falls as they are made warmer. This results from the interaction of molecular activity (which is temperature) and the electrical forces that tend to hold liquids together. At higher temperatures, molecular activity is more able to overcome those electrical forces. But as the liquid cools, its "holding-together" forces gain the upper hand, making it more viscous.
At the pour point, solids are forming in the oil. Waxy crystals form (which can block filters or resist flowing through small oil passages during cold start and warm-up). At temperatures a few degrees lower, the oil solidifies as a whole. People who have outdoor tanks for heating oil and those who must start diesel engines in very cold weather are familiar with waxing. When cold weather arrives, they add a solvent that keeps waxy stuff in solution. The range of pour points for mineral lubricating oil base stocks is -20 to -37 degrees Fahrenheit, while for synthetic Group IV oils made from polyalphaolefin base stock, the range is more like -40 to -57.
Very few of us plan to operate our motorcycles at such temperatures. One winter when I did so, I tried to pull off a spark-plug cap and its plastic shattered in pieces.
What determines pour point? Oils consisting of long straight-chain molecules have higher pour points because each such molecule is in contact with many others, impeding their movement and thereby increasing viscosity. Therefore, when lower pour points are desired, more compact branched-chain molecules are the choice.
Pour-point concerns have a long history. In 1905, waxy components in lube oils caused them to solidify even in moderately cool weather, making it impossible to hand-crank engines for starting. The first response was to fit little petcocks to each cylinder and, before attempting to start, pour a few cc of gasoline through them into the cylinders. The early motorist then waited as the gasoline dissolved the solidified oil, then continued with the long and arduous starting drill. If large aircraft piston engines of the 1940s had to be started in very cold weather, their lube oil was diluted after shutdown with aviation gasoline, reducing its cold viscosity. Upon starting and warm-up, the gasoline evaporated to atmosphere from the hot oil.
The next measure taken against oil solidification was to dewax it. One method was to cool the oil enough for the wax to form, then filter it out. Another, solvent dewaxing, required the addition of a solvent specific to the waxy component, which could then be separated from the oil.
The present trend in gasoline engines is to make ever-smaller engines work harder. This improves fuel economy because the smaller the engine, the lower its friction at highway speeds. The necessary on-ramp acceleration is then provided by turbocharging. This produces more severe engine-operating conditions. As API (American Petroleum Institute) standards for gasoline engine oils become ever more stringent, makers of petroleum oils have abandoned physically removing the waxy stuff and now chemically transform it into a molecular species with a higher pour point. This is done with a catalyst, which is a substance that accelerates the rate of a desired chemical reaction without itself being changed by it. I like to think of catalysts as “mugger’s helpers,” who hold the target molecular species for a moment in a way that favors the desired reaction. In some cases, the catalyst (typically an atom of a heavy metal, which has especially strong electrical fields near it) is located inside a molecular “cage,” whose shape selectively admits the target molecules while excluding most others.
Improvements are continuous in such areas for it’s obvious that transforming waxy species into liquids of lower pour point is less wasteful than discarding them. In the past 20 years, in one major producer of lube oils, there have been five generations of improved catalysts just for this process. When I was a little boy and my family drove through New Jersey on our way to see the grandparents in Indiana, I saw “unwanted refinery streams” being flared-off from tall burner stacks. Today such inefficiencies cannot be afforded.
The pour-point ranges given previously are for oil base stocks, before additives are mixed into them. A class of additives called pour-point depressants works by forming a molecular monolayer around any waxy particles that form, preventing them from clumping together or sticking to surfaces. I was thinking of this monolayer formation earlier today as I washed up some things in the kitchen. When I rubbed a dish with greasy residue on it, some residue was washed away by hot water but most of it stayed stuck to the dish. But when I added a drop of detergent, a non-stick monolayer instantly formed around every bit of residue I rubbed loose, preventing it from sticking back onto the dish. Instead, the running hot water sluiced it away.
Ideally, before starting any engine its oil system would be pressurized, either by an electric pump or by a pressure accumulator. Most of us are satisfied to let the engine oil pump do the job in its own time. Every engine oil system has a “last point to be reached.” Older riders will remember that certain Honda V-4s sold in Europe had camshaft problems resulting from particular cam lobes being the last to receive oil, some four minutes after start-up. Kits of larger-bore external oil lines were marketed as a fix, accelerating the flow of sluggish cold oil.
And there is the advice given by Neil Spalding of Sigma Performance to would-be buyers of used Ducatis: “Ask the owner to start the bike. If he touches the throttle at any time in the first minutes after the engine starts, just say, ‘No, thank you,’ and go.” The reason? Revving the engine before oil has reached the farthest reaches of the oil system operates some elements of the valve system without fresh oil, risking scoring of lobes and cam followers.
I have a photo of MV Agusta's long-ago racing manager, Arturo Magni, heating engine oil on a portable stove prior to it being poured into the engines of factory racebikes at a 1960s German Grand Prix. Oil whose viscosity had been lowered by heating in this way would quickly fill the oil system, preventing potential parts damage. Fortunately for us, low-viscosity, low-pour-point base oils plus pour-point depressant additives (mostly) make such measures unnecessary today.
