After checking valves (first order of tune-up procedures), ignition maintenance and carb/fuel checks, there isn't much wrench-turning that is left to do. However, I think that most F/2F will benefit from cleaning in two areas, the intake and exhaust ports in the head, and the piston rings. Also, a borescope for cylinder cross-hatch (cylinder-hone) or peek into the head ports, will tell you tons about the engine's condition and its needs.
When oil gets too hot, lets say it is leaking thru the valve guides more than it should (head intake/exhaust ports), or, it made its way thru the oil control rings due to cylinder wear, ring wear, or because it was too high a viscosity oil for the ring tension, it polymerizes, or becomes like seasoning on a piece of cast-iron-cookware. Really, the solution is to do an engine tear-down. Piston rings that are stuck to the piston groove don't perform correctly and can lead to even more blow-by. A valve caked with thick layers of baked-on 10W40 isn't going to breathe thru the port properly. And who knows if valve-congestion makes valves stick, or overheat from glowing carbon burning? However, there is a detergent package in the crankcase oil, and additives are commonplace in gasoline fuel. So, fresh gasoline is always a must. I like the Techron additive(s) for gasoline. I no longer have a Chevron filling station to buy from, so I buy bottles of additives. I did Lucas last summer, right now Marvel Mystery Oil (super inexpensive) is in the fuel tank. Techron is either for long-term-storage, or they make one for use right before an oil change; I always forget which is which. Because oil has detergent, I do an oil change (w/ an conventional API-SP) about every three or five tanks of gas, as it is probably fuel-contaminated (blow-by, or rich-condition from a cold or hot carburetor) or the oil is water-contaminated (blow-by and resulting H2O-condensation). BTW, high-viscosity oils have longer hydrocarbon chains and are more subject to thermal break-down, so I stick to a season-appropriate viscosity (referring to the Owner's Manual). I never use oil additives (Marvel Myster Oil is in the fuel tank-only). I often change the oil without also changing the oil filer. I refuse to just think that oil is oil and gas is gas when there are resources being made available to the consumer that claim to address a common problem - engine deposits, but, I could be wrong about all this liquid tune-up stuff for running old engines, as I'm no expert.
