Going back to the OP's original questions, first the oil mess. You're stuck with it because it's inherent in an engine that the oil has to pass through it without getting burnt inside. Reducing oil content by a few percent only makes a miniscule difference to the amount that will collect on the model. Synthetics are easier to wipe off but they're far less tolerant of an incorrect (too lean) mixture setting. Castor is harder to clean off because it's so sticky but it's also just as sticky inside the engine where it does its job. OK, that's way simplified but an indication of why castor is great stuff for lubrication.

As far as nitro content goes, so long as you don't use so much that the engine detonates then you can use whatever amount you like for running in or flying with. All engines (and I include even the little .049's) run perfectly well with zero nitro but nitro will add some extra power and possibly give a better idle. ASP engines tend to have a somewhat higher compression than most engines and that limits the amount of nitro they'll tolerate before detonating. It also means they'll handle zero or low nitro fuels better than some others. Anytime you change fuel types the engine must be retuned, even if you just change brands.