I'm not sure that, generally, people are aware of what's happening to oils and additives and such. I just looked at my gallon jug of Mobile Delvac. It's rated SM, SL for gasoline engines now.

The ZDDP package is there for extreme high pressure contact points within the engine, mainly the camshaft and flat lifters, like we find in the Airhead engines. A lot of modern engines have evolved to roller lifters or radius-faced lifters which don't have that immense contact pressure of the older flat 'tappets'. A lot of modern oils have a synthetic supplement that does pretty good as a zinc replacement. So oils AND engines are evolving due to the EPA requirements.

I have heard that years ago when the first diesel VW Rabbits were introduced, Castrol said their GTX oil was "C" and "S" rated. Many VW diesel engines were harmed and VW sued Castol for misrepresenting their oil. So picking the correct oil does make a difference.

To those diesel truck enthusiasts, Ford Motorcraft 15w40 does not have a SM, SL rating. Only a CJ-4 plus. Specifically, warns against use in cars with catalytic converters. It may have to ZDDP package. Not sure what the "plus" means. May be an option on airheads, but I don't know for sure.

I hope members get some use out of this thread. At least to start paying attention to the back of an oil bottle.

This why everyone hates oil threads. Now, were are discussing motor oil for diesels? Please forget about using "C" rated oil in an airhead. You can not know what you are getting. Do not put a motor oil in you motor that does not indicate an API service rating of SF or SG,....period.

You know eventually these will dry up and not be available

..."You know eventually these will dry up and not be available"...

This is speculation, at best. Like anything in a market economy, if there is a demand for a product, some enterprising vendor will meet that demand.

Castrol GTX used to be the "go to" motor oil for airheads. Since GTX is mass market motor oil for modern automobiles the API rating reflects the needs of modern automobiles. Castrol also offers a 20W50 labelled as type A or suffix A. You have to ask for it, and not everyone stocks or sells it.
Bottom line; if there is a demand for any type of product, someone will supply that product for a price.

I only use Golden SPECTRO _ 4

I've also have used this in my 247 & other engines for a while since the oil rating changed. 5-6 tablespoons / 1/4 cup ... is enough to add to a 3-4 qt change
https://lucasoil.com/products/engine-oil-additives/engine-break-in-oil-additive-tb-zinc-plus

you will get some understanding from this link:

https://www.amsoil.com/newsstand/classic-and-vintage/articles/reduced-zddp-and-wear-protection/

Protects camshaft lifters and valve train during break-in period of motor
Excellent for flat tappet camshafts during break-in or as an
additive to any motor oil to prevent premature wear
Exclusive blend of extreme pressure additives
Add one bottle with every oil change to increase the
zinc content of your motor oil
Designed for race applications requiring additional
extreme pressure additives
Dyno testing shows a more efficient break-in when used
resulting in less engine run time prior to race day
Not designed for passenger car use

The one oil always recommended by Oak was Spectro Golden 4 20/50. Semi synthetic, high zinc content, ability to top up with whatever you could find when traveling. Check Oaks tech articles, he knew his stuff! RIP Oak

Just run Golden or Platinum Spectro, both highly recommended by Oak and Snobum My 83 R100 RS has been on Golden Spectro 20w 50 all of its life. I have been running 15W 50 Platinum last few years with no ill effects. It definitely starts a little easier in the cold with the 15w -50.

If you are into synthetics, Amsoil MC 20-50 is also rated SG and Mobil 1 15-50 is recommended for flat tappet engines. So either would work fine.

You know eventually these will dry up and not be available

This is very true.

The 1981 R100RS model dried up and became unavailable decades ago ! 😛

Oil is a contentious issue with vintage vehicles but I have had very good luck with 20W50 Valvoline VR1 Racing Oil in all my vintage bikes. It has a good concentration of zinc at 1300 ppm which may or may not be important for vehicles with flat tappets.

https://540ratblog.wordpress.com/2013/06/20/motor-oil-wear-test-ranking/

This guy is the Snowbum of motor oil. Keep an open mind and read it all. It is lengthy but gave me a better understanding of motor oil.