30-09-2019, 01:39 AM
(This post was last modified: 30-09-2019, 03:18 AM by Bob Culver.)
Many factors influence oil choice; some of those relevant pre war barely apply to moderns with effective seals and quiet all helical gears. Oil was chosen to reach plain bearings if any, quieten, reduce leakage, reduce power loss, facilitate synchro (thinner). Crash change requires a thick oil to slow for upward changes but a thin one to facilitate downward. And what suits changes at low temp may not suit when hot.
Except possibly the boutique monograde classic oils, all now contain anti wear and or EP additives and SAE 20 or thicker fully adequate for lubrication. Many other cars of the 1950s which ran engine oil SAE30 in gearboxes later recommended 20W50 for the same gearbox. The reference temp for W oils is below 0 degC . According to graphs in the book Which Oil the crossover point for 20W50 and 30 is still below 0 deg C, so bulk SAE20W50 is notably thick at normal use temps. The multigrades thin out less as temp rises. With 20W50 and 15W40 cheaply available these seem fine in place of monogrades now these have become expensive.
Gear oil classification is different but SAE90 approximates SAE50 engine oil, and 85W is somewhat thinner. The grades cover a range so comparisons can vary with brands. Mainly because it was used in many other cars, many Sevens ran EP90 gear oil in the 1950s and 60s. (Many now avoid EP oils in diff or gearbox for fear of bronze/brass corrosion but the major oil makers claim OK and doubtful if sufficently high temps reached anyway. Austin publications in 1950s recommended for diffs. Dunno if anyone has ever reported a positively identified case of excessiev wear.)
Except possibly the boutique monograde classic oils, all now contain anti wear and or EP additives and SAE 20 or thicker fully adequate for lubrication. Many other cars of the 1950s which ran engine oil SAE30 in gearboxes later recommended 20W50 for the same gearbox. The reference temp for W oils is below 0 degC . According to graphs in the book Which Oil the crossover point for 20W50 and 30 is still below 0 deg C, so bulk SAE20W50 is notably thick at normal use temps. The multigrades thin out less as temp rises. With 20W50 and 15W40 cheaply available these seem fine in place of monogrades now these have become expensive.
Gear oil classification is different but SAE90 approximates SAE50 engine oil, and 85W is somewhat thinner. The grades cover a range so comparisons can vary with brands. Mainly because it was used in many other cars, many Sevens ran EP90 gear oil in the 1950s and 60s. (Many now avoid EP oils in diff or gearbox for fear of bronze/brass corrosion but the major oil makers claim OK and doubtful if sufficently high temps reached anyway. Austin publications in 1950s recommended for diffs. Dunno if anyone has ever reported a positively identified case of excessiev wear.)