Jianis, et al.,
Here is a link to some information on
motor oil viscosity that may be useful. Click on the "SAE J300..." link in the link to see the viscosity specifications for the SAE grades. Note that, at 100C, the viscosity of the 20W-50 grade oil will be about 75% greater than the viscosity of a 30 grade oil (either a straight grade 30 or a 10W-30). The 20W-50 grade oil meets the low temperature cranking requirement at -15C (+5F) and the pumping requirement at -20C (-4F). The straight 30 grade oil does not need to meet any low temperature requirement; it may be too thick to pump or crank at below-freezing temperatures. Note that there is no 30W grade oil in the SAE standard, only a 30 grade. There is a 20W and a 20 grade oil; it is the only oil grade number that shares both a "W" (for winter?) and a non-W designation. The "W" grades all need to meet a minimum viscosity requirement at high temperature (100C or 212F), but the non-W grades do not need to meet any low temperature requirement.
When I had my first service station job in 1957, we stocked oil in the 10W, 20W-20, 30, and 10W-30 grades.
With regard to the "breakdown" of the viscosity index (VI) improvers of a multi-grade oil, here is a link to
a discussion in another forum that presents some real data. The VI improvers in some of the samples "broke-down" worse than others. The worst case dropped the viscosity of a 15W-40 grade oil at 100C from the middle of the 40 grade range to the upper end of the 30 grade range.
I'm not sure if all of this answers the original question of what oil to use in your 216. Is this the Castrol 20W-50 oil
for Classic Cars? The viscosity may be higher than you need at engine operating temperatures, but the 20W cold rating would probably be better for cold starts than a straight 30 grade oil.
Hoyt
