WHICH OIL? Sir,
Not for the first time, my attention was recently drawn to a full-page advertisement extolling the virtues of a well-known brand of oil which seems to claim the same viscosity under all temperature conditions; at least the advertisement argued that " a thin winter oil is too thin when hot " and " a thick summer oil is too thick when cold "—so that the brand advertised presumably became neither the one nor the other under sin-filar conditions.
This brought to mind my efforts to retain a decent oil pressure in a highly-tuned Ford 93A engine, fully reconditioned and carefully run-in, which was fitted in a space-frame chassis I owned some little while ago. Using a good grade oil of SAE 30, the normal oil pressure of 30 p.s.i. fell away to less than 15 p.s.i. within half an hour of reasonably hard road motoring. Realising that the sump capacity of just over five pints was proving to be insufficient to transfer heat with the necessary rapidity, the sump was enlarged by welding on" blisters " to increase the capacity to 12 pints. Result on SAE 30 oil-30 p.s.i. on starting, falling away to below x5 p.s.i. within 45 minutes on the open road. The fitting of a non-standard high-capacity oil pump giving 65 p.s.i. on starting effected no improvement—the pressure remaining at that worrysome 15 p.s.i. when hot.
Then I had a brain wave—abnormally high oil temperature being the obvious cause of the trouble, thus unduly thinning the oil, the answer was, of course, to use an oil which "was not thin when hot," not " too thick when cold "—which would surely give some improvement. And so 12 perfectly good pints of SAE 30 were jettisoned, to be replaced by 12 pints of the " new idea in oils." Result-65 p.s.i. on starting, 15 p.s.i. within 30 minutes of fast road work. The fitting at this stage of an oil-temperature gauge showed the temperature at take off to the gallery to be 19o° F.—so where do we go from there so far as special oils are concerned ? It will no doubt be argued that there arc temperature limitations above and below which static oils cannot be expected to retain their properties—but I have yet to see an advertisement confess the fact.
The fitting of an oil cooler cured the trouble, the oil pressure remaining steady at 25/30 p.s.i. thereafter, both with ordinary SAE 30 oil and with the multi-grade brand, and if this result proves nothing else it is sufficient to convince me that these special oils can also become "too thin when hot." In my youth I recall that one heaved in good old Pratts oil at is. per quart, and no special or miraculous claims were made for it, either; but in a score of old bangers driven permanently to their limit I do not remember one bearing failure, other than on a dry-sump Aston Martin when the return hose from the scavenge pump to the oil tank split, so that all the oil ended up in the road instead of back in the tank. I still think there's a lot of malarkey published about oils today; that they are too expensive at 2S. id. or more per pint; and that
I always feel I've secured a better bargain when I've paid two bob for my MOTOR SPORT each month.
Brentford. R. W. BURNELL.