Yes, everyone has an opinion on oil.  3000 mile/6 month maximum change
interval with 15w-40 dino oil worked well for the 225k miles I put on my
previous '84.  It was still running fine when I swapped it out for a 2.1.
Just don't use the modern oils like 5w- or 10w-, they are too thin for the
loose waterboxer and you won't get enough oil pressure at high temps.

Another good idea is to use "top tier" gas, at least every third fill up,
since they have more detergent additives.  See www.toptiergas.com  A
constant diet of cheaper EPA minimum quality gas (Arco, Costco, Sam's,
Safeway, etc.) can lead to carbon buildup on the intake valves which can
cause a stuck valve.  No need for premium gas for our low compression
engines.

You might find it interesting to do an oil analysis before your next change
to guage the engine condition. I've used www.blackstone-labs.com for years
on all my vehicles.   It's particularly useful for assessing bearing wear
and finding coolant in the oil.

Whatever you have been doing has certainly worked, but start saving for that
rebuild!

Stuart