I forgot to mention earlier, that performance-wise the old K&B carb and Perry carbs are about equal for all intents and purposes.

I actually think the stores push synthetic oil fuels as with higher nitro percentage fuels, it causes the engines to rust out on the inside faster. So they get to sell more engines. Most people don't repair the engines, they just buy new ones instead. Plus keeping one type of fuel reduces their inventory costs as well. But like you stated, they probably do not see guys running older engines much if any nowadays.

Being way up there in MN makes it tough, no easy solution for you there. Being way down here in Texas, where he have a almost year round flying season, gives us more choices in fuels to use. Plus PowerMaster is made here in Texas too.

But with the K&B Sportster engines castor oil is really needed due to the aluminum cylinder. The castor oil tends to form a varnish like coating that the engine designers were taking advantage of to prevent the aluminum cylinder sleeve from wearing out prematurely.
Other older more vintage engines need castor oil too. McCoy and Testors is one example, they have plain steel unhardened cylinder sleeves that wear out fast without castor oil. Many old bushing engines with iron pistons actually need the castor oil, I remember guys trying synthetic oils in a old engine, only to have it's compression go away, as the engine wore off the castor oil varnish layers that was letting it work good.