First off, I do NOT base clean. Maybe I've just never had a dirty base?!?
Second, Kerosene is the suggested cleaner for motorcycle chains. It's strong enough to loosen gunk, but not so strong that it ruins rubber o-rings.
If I did have to clean, I'd be tempted to use dish soap and hot water first. My guess is only tree sap would be tougher than dish soap, and you could use almost anything for tree sap. Mineral spirits, alcohol, etc.
I still say though, what's all this fuss about base cleaning and hot scraping?!?