It may be noticed that every oil company advises the motorist to empty his engine sump fairly frequently and refill with fresh oil—say, every 1,000 miles during a car’s first year and subsequently every 1,500.
One is frequently asked by cynically-minded readers whether this advice is not merely sales propaganda.
Frankly, no. There is a perfectly sound and sufficient reason. It is not that the oil itself becomes “worn out” and so useless, but that it becomes contaminated.
“This occurs on even the best regulated car” says a technical expert in the Shell Research Department. ” There are five main sources of contamination—unburnt petrol in the oil, condensation of water vapour, particles of carbon, grains of road dust and small quantities of metal worn off the moving engine parts.
“Petrol, of course, thins down the oil it mixes with, and so in time impairs its lubricating qualities. “Then, water vapour is a normal product of combustion.
This is why on cold mornings you can sometimes see water dripping from the cold end of the exhaust pipe.
“Some of it—a good deal, if the pistons, rings and cylinder bores are at all worn—finds its way, like unburnt petrol, past the piston rings into the crankcase and sump. So also with particles of carbon from the cylinder head, and with road dust admitted through the carburetter.
“If these impurities are allowed to remain in the lubricating oil they will circulate to the bearings and cylinder bores where they are bound to have an abrasive wearing effect.
” The only remedy is to drain off the old oil—being careful to do so when it is hot, after a run, so that it flows freely—and refill with fresh oil. Years of research and practical experience with our own very large fleet of cars and commercial vehicles have shown us at what intervals this ought to be done.