Further to Mr RW Burnell’s letter in the April issue of Motor Sport entitled “Which Oil “, I am rather surprised that he expects to obtain a much higher oil pressure with a “Multigrade” oil than with a standard SAE 30 oil.
If we consider, for example, a 10W/30 oil, which seems to be the most generally used Multi-grade oil in this country, this oil is developed to meet the viscosity specification of an SAE 10W oil at 0 degF, and an SAE 30 at 210 degF. It can, therefore, be assumed that at normal engine operating temperatures of 180-200 degF that the viscosity of the 10W/30 and the standard 30 oils will be near enough the same. I must add, however, that two oils of the same viscosity will not necessarily produce the same oil pressure. But even so the difference will not be as great as to 10 lb psi and more which Mr. Burnell seems to expect.
The advertisements which are published advocating that an oil “remains thick when hot and thin when cold” are rather misleading and the blame for this, I feel, lies with the publicity boys. The majority of sales people employed by the oil industry today know next to nothing about oil and they play on the fact that the average motorist knows nothing. Even quite elementary questions put to some oil salesmen will be “referred to our oil chemist!”
MK Smith, Rumney.