With my recent birthday, I am now closer to 85 than to 25, so maybe I can shed some light on the beliefs of older mechanics.
Mobil 1 was first marketed in the early 1970s, I recall. They blatently advertised that you could go 25,000 miles between oil changes. The earliest formulations were somewhat aggressive with rubber seal materials still on the road in some cars. The only viscosity initially available was 5w-20 This combined to turn a lot of people off to synthetic oil.
Other than Mobil 1, other major oil companies did not heavily market motor oil. The brands you normally saw in the auto parts store were Quaker State, Pennzoil, Valvoline and Castrol. Some people thought Castrol was made from castor beans, which it was not, except for some specialized racing oil they made (and you would not want to use, because it would get tacky over time). Valvoline made a "Racing oil", single grades going up to at least 60 weight, and a 20w-50. Multi-grade oils were still not commonly accepted, and some made their own multi-grade oil by adding STP Oil Treatment to single weight oil.
Back in those days, Valvoline heavily advertised and promoted racing. So a lot of people used it. The only person I know who used "diesel" oil in a car was my father, who was ahead of the curve and used Chevron RPM Delo.
People recall one bad experience that occurred years ago and that forms the basis of their opinion today. For example, I remember in the early 1970s I used some Pennzoil in a 750 Honda on a hot cross country trip, and more of it disappeared from the dry sump tank than my usual oil, Castrol GTX. I havn't used Pennzoil since.
The last thing mechanics want is an irate customer coming back to complain. So they recommend what never gave them any trouble, and they do not recommend what they have heard bad about, no matter how long ago it was.
There is an old saying: "You can always recognize the pioneer....he's the guy dead by the side of the trail with the arrow in his back".