With that mileage I'd be using a 60 weight, like Millers, when these engines were thrown together the tolerances were nowhere near as tight as todays engines, that NEED a thinner oil a modern engine has around 3 thou bore clearance, the alloy pistons on modern engines, a lot also contain a massive metal inner part and are known as steel strapped, these have a 1 to 1 1/2 thou clearance, the reason for the steel strapping is so they don't expand quicker/more than the bores walls, as they would seize otherwise, this is one reason for thinner oils, another being emissions and economy.
Mobil 1 will be drunk big time, as it thins way too much, Pro S is the best really for normal money, although FUCHS is better, my choice would be SUNOCO RACING if you can get it.
I use pro s as it is easy to get and good price, get mine from matt lewis.
tabetha