We used to use Marvel Mystery Oil on large Radial aircraft engines when we had a coked up guide.
A gallon or two of MMO in the oil (30 gallon useable tank) made the sticking guide clean up almost immediately (it would clear up on ground run).
I've run low doses of it in old engines to clean them out (couple ounces per oil change, change the filter every 400-500 miles if you try it though
).
