[40 Deluxe]

Remember, most roads in the A's heyday ('30's-50's) were gravel or plain dirt, so lot's of dust kicked up. Now, does anybody even drive on a gravel road? Cleaner air plus far better detergent oil today means you can go a lot further.
Instead of dirt, the biggest limiting factor is condensation and blow-by (acid forming) contaminants, which an oil filter does not remove!
The worst thing for oil contamination is failure to completely warm up the engine whenever it is used. For example, the person who goes out and starts his A every week in the winter and runs it only 10-20 minutes is doing far more damage than driving it a 1,000 miles.