Engine Oil - Saybolt vs SAE

I received a very nice reply to my question about oil viscosity measurement from Andrew Poole of the Penrite Oil Company. I have pasted it below and have attached the accompanying chart.

Comparison of viscosity classifications

Firstly, my apologies for not answering your question more quickly.

I have attached for you a comparison chart which shows approximate equivalents of the two major measures of viscosity (Kinematic and Saybolt) and a number of viscosity classifications systems. Please note that they are only approximate but should serve their purpose for you.

In looking at the Stutz recommendation I would suggest you look only at the viscosity at 212ºF that being the equivalent temperature to that used to specify the SAE grades (both engine and gear).

You will note that the temperature in the chart for Saybolt viscosity is 210ºF not 212ºF but for the purposes of this exercise the variation of 2ºF will not make a difference.

To read off the equivalent SAE viscosity take a horizontal line from the SUS at 210ºF through the SAE grades from which you will see that in summer Stutz are recommending what is now a heavy SAE30 and for winter what is now a borderline SAE20/SAE30.

Similarly if you had a gear oil recommendation of, say, 160 SUS at 210/212ºF you would take your horizontal line across to the SAE Gear Oil column which would indicate an SAE140 should be used.

Ignore the other two viscosity classifications - AGMA is for industrial gear oils, ISO for other industrial oils and have no application to your type of use.

Note also that whilst Kinematic Viscosity is a modern universal viscosity measurement, Saybolt is essentially American. A car of the same age as your Stutz but made in the UK might well quote a Redwood viscosity whilst if it came from Europe it might be an Engler viscosity. The latter two types of viscosity are rarely referred to today and their conversion is much more convoluted.

Trust the above is of use and interest to you.

In relation to your question on the worm drive oil, the comment in the Stutz literature is correct for its time but not necessarily now.

Penrite Transoils are straight mineral oils, that is they contain no additives. Refining techniques have improved markedly since 1926 and one of the results is that today's oils have very low sulphur content, and that which is present is chemically inactive, so that it does not attack yellow metals. Transoil is therefore absolutely safe to use in your worm drive.

Where care is needed is where an EP additive containing oil is contemplated for use in a worm drive (such as a modern hypoid differential oil). Some of these will attack the bronze although others may not, it depends on the chemistry.

However, it is only with later worm drives where the loading on the gears is sufficiently high to need an EP additive containing oil, eg. Peugeot continued using worm drives way past the time most other manufacturers had turned to hypoid drives, and these do require an EP additive containing oil.

Again, trust this is of use and interest to you.

Andrew Poole

Penrite Oil Company