Viscosity #2

[Daryl Smith]

Opinions differ, even among professionals in the oil business, as can be read in this article from Motor Trend. From all I have read I wonder whether the endorsement of 20/50 oil in the article is simply to protect a profitable market, which would be par for the course for both the magazine and marketing arms of the manufacturers. Again, an opinion is just that, an opinion. These are supposed professionals in the business, and they can’t seem to agree.

Motor Trend is based in El Segundo, California…and climate is taken into account in some of the opinions.

Below are 2 of the opinions which align with much of what I have read. YMMV.

https://www.motortrend.com/how-to/0702sr-engine-oil/

Ed Newman: For gasoline engines, 10W-30 or 10W-40 are the most commonly used viscosities and will apply to practically all applications.
For many years, 10W-40 was the most common viscosity grade. The need for higher-viscosity oil arose from, in part, manufacturing technology of the time, causing loose engine tolerances, and engine oil quality was low compared to modern standards. Thicker oils will help to reduce oil consumption, wear, and to keep the oil pressure higher. These features are of particular importance in worn engines. If the vintage engine has had a high-quality rebuild performed and it is not modified for greater power, there is little need for thicker oils. Using 10W-30, for example, is fine.

Cameron Evans: More viscosity can certainly help for a true "vintage" engine with very wide clearances and oil pressure issues. However, we find that many street rodders commonly select an oil viscosity that is too heavy. Unless one is looking at .005-inch clearances or very worn rings, a 20W-50 is probably not required. Running such a heavy product usually leads to excessive wear at cold temperatures and gives up fuel economy. In many cases, it is possible to move down one oil weight (from 20W-50 to 10W-40) when switching to a fully synthetic product. The rule of thumb is to start with 10W-30 motor oil for crate engines, homebuilt engine projects, and the like. In a hot climate, 10W-40 is a better idea, and 20W-50 motor oil is only necessary in extreme conditions or very powerful engines.

Here’s a chart from ‘Driven’ (AA Oil Company) that takes in both oil temperature and bearing clearances to choose the right weight oil.
https://aaoil.co.uk/driven-racing-oil-b ... clearance/

[Daryl Smith]

For oil the 'W' does not mean 'weight'!

https://www.youtube.com/watch?v=48pviPLgaPQ

[Daryl Smith]

At one time diesel oils were recommended as an affordable oil for flat tappet engines, seems that may not be such a great idea.
In case you didn't read it from the other thread...

https://www.speed-talk.com/forum/viewtopic.php?t=67124
Page 3, 5th post down:
" Oils like Rotella have ~50% more ZDDP though it's typically a different type of ZDDP that's catered to a diesel engine's operation. It's good for a diesel that lugs at 1500-2000 rpm for hours unend. Aside from the ZDDP, there's a decent amount of detergent, a load of dispersant (for soot control), and very little of anything else. The anti-foaming / anti-aeration additive content is ~30% lower, the EP additive content is ~70% lower, and it contains no friction modifiers."

[ADR]

Perhaps relate to this discussion is the switch from flat lifters which slide on the cam lobe to roller lifters and other modern solutions. The oil of yesteryear included since and was well suited for the sliding wear scheme, whereas modern oils are better suited for the rolling schemes.

The elimination of zinc from engine oil was triggered by the introduction of O2 sensors and catalytic converters - which foul with such additives.

FYI: https://help.summitracing.com/app/answe ... is-zddp%3F