Another early test of Mobil 1 was said to be in their pipeline unit trucks. Pipelines are long and to service them requires a lot of driving. I don't know what Mobil used for a standard during the early 70s but I do know Texaco would drive their trucks to a minimum of 250,000 miles before replacing them.

Anyway... It is safe to say that the majority of Mobil's trucks had some miles on them. When they tested them with Mobil 1 they leaked from every seal. The cause was tracked down to the very nature of Mobil 1 and why it worked so well as a lube.

The molecules of syn oil are tiny. The molecules of conventional oil are large. The gaps in the seals, such as the forward crank seal, due to normal wear would not allow the larger molecules of conventional oil to pass through. The tiny ones of syn would slide right on out of the gaps due to wear.

So the decisions was made to not run it in higher miles cars.

I think that over the years oils have been reformulated and this is why you see some oils marketed as 'higher mile engine oils'.