on older engines in an era gone now, folks used to run 30 weight on anything with over 70-80K miles (before wide spread use of multi weight oils). same reasoning as those using 20W-50 in their stangs.
i use 20W-50 for 9 months, and 10-40 in the "winter" (winter in Tucson means highs of 60* instead of 110*).
for a mill with some miles on it, the tolerances are not what they used to be; thicker oil should be fine (IMHO) if a less viscous oil is allowing burning, less pressure than is liked, etc.
you really want the thinnest oil that keeps some viscosity when at its hottest. all new cars (most of them) have specified 5W and 10W-30 oils. thinner, as mentioned leads to better economy, which is necessary for various reasons when manufacturers have the vehicles tested.
now i know some new cars have really small clearances and PASSAGES. thicker oil might be ok for worn tolerances, but you have to be conscientious of the passages that the oil must flow through.
i personally like thicker oils. i hate the 10W-30 oils. anyone whom has changed the stuff out of a car when it is hot, knows it is about as viscous as water. my brain does not like to see that. remember, i fight viscosity breakdown and thinning in my climate. i still run 10-30 in some of the newer cars. i will likely continue until they start burning or pressure drops off.
Driftin, i would not want to go with too long of an extended OCI on the 5W-50 you use. the larger than normal difference in the viscosities leads to more shearing. i do run the same stuff in a car of mine, which i start every few months and drive to a car show once a year. nice flow when cold, and theoretically holds up decently in hotter temps.
this is all just my two cents, based on what i know and have read. i could be wrong, but that is one reason to post this; so people can refute it and we all learn. good luck to all.