Those original synthetics were ester and PAO only oils.
The stuff sold today as synthetics are different in two ways. You shouldn't have those problems with better 'synthetics' today. That's not to say you ought to try them. Bad experiences like that are a powerful incentive!

You may be able to figure it out what the base stock(s) are from the material data sheet of labelling.
When you read Widman's paper you'll see that's where he gets the info, although sometimes he has to figured out based on clues in the documents.

here's a some basic overview of the base stock snipped from https://www.machinerylubrication.com/Read/29113/base-oil-groups

API Base Oil Groups
In the early 1990s, the American Petroleum Institute implemented a system for describing various base oil types. The result was the development and introduction of base oil group numbers.

Group I base oils are the traditional older base oils created by a solvent-refining technology used to remove the weaker chemical structures or bad actors (ring structures, structures with double bonds) from the crude oil. Solvent refining was the primary technology used in refineries built between 1940 and 1980.
Group I base oils typically range from amber to golden brown in color due to the sulphur, nitrogen and ring structures remaining in the oil.

Group II base oils are created by using a hydrotreating process to replace the traditional solvent-refining process. Hydrogen gas is used to remove undesirable components from the crude oil. This results in a clear and colorless base oil with very few sulphur, nitrogen or ring structures.
In recent years, the price has become very similar to Group I base oils. Group II base oils are still considered to be mineral oils. They are commonly used in automotive engine oil formulations.

Group III base oils are again created by using a hydrogen gas process to clean up the crude oil, but this time the process is more severe and is operated at higher temperatures and pressures than used for Group II base oils. The resulting base oil is clear and colorless. [I]t is more resistant to oxidation than Group I oils.

The cost of Group III base oils is higher than Group I and II. Group III base oils are considered mineral oils by many technical people because they are derived directly from the refining of crude oil. However, they are considered synthetic base oils by other people for marketing purposes due to the belief that the harsher hydrogen process has created new chemical oil structures that were not present before the process. It has synthesized (created) these new hydrocarbon structures.
Group I, II and III base oils basically reflect the evolution in refining technology over the past 70 or 80 years.

Group IV base oils are polyalphaolefin (PAO) synthetic base oils that have existed for more than 50 years. They are pure chemicals created in a chemical plant as opposed to being created by distillation and refining of crude oil (as the previous groups were).

PAOs fall into the category of synthetic hydrocarbons (SHCs)...and are significantly more expensive than Group III base oils due to the high degree of processing needed to manufacture them.

Group V base oils comprise all base oils not included in Groups I, II, III or IV. Therefore, naphthenic base oils, various synthetic esters, polyalkylene glycols (PAGs), phosphate esters and others fall into this group.