As far as I know the only difference between 2 cycle and 4 cycle fuel is the (castor) oil content.

Can you use 4c fuel in a 2c engine, with the understanding that there will probably be more slobber to clean up at the end of the day?

Can you use 2c fuel in a 4c engine if you add some kind of oil to it?

Your are right on both counts. The required percentage of oil for 4 strokes has been debated for some time. O.S. said to just use 2 stroke fuel although their engines will perform better at idle (for example) with 12 to 16% oil.

One thought for all of us here: How many engines have you actually worn out? I can think of only one in my case (an OK Cub) that was obviously a lemon as it was running on 20% castor oil fuel in cool and non dusty conditions.

I've been using 10% 2-stroke glow fuel in my saito and OS 4-strokes for about 14 years with no problems. Same fuel I use in my 2-strokes. I tried the more expensive 4-stroke saito/YS 20/20 fuel, and naturally it ran a little better, but the cost just wasnt worth it. I dont tune my engines for every bit of power I can squeeze out of it. I have four saito 1.20's and two with well over 200 hours run time still with all original parts, including bearings. I tear them down around every 60 hours for a good cleaning which includes taking the valves out and cleaning all the carbon out including the intake and exhaust ports.
Edwin

Forgot to mention, its Power Master.

Most 2 and 4 strokes engine do fine with 18% oil. If you don't want to use different fuels just use something with 18% oil.

Greg

Power Master changed the formula on their 10% 2-stroke a few years back (when they were sold). It used to be 18%, now it says 17%. Havent noticed a problem in two years of use.
Edwin

Sure, 1% isn't really going to make a difference, needle setting has more influence. I run low oil content in most my engines. But I don't recommend it to others, it just draws blank stares.

OS recommends 18% oil in all of their engines. How many of their car engines get run on 18% oil? Probably few to none. Most car fuels have 10% oil.

Greg

I use 18% oil fuels in my 4S engines and ball bearing 2S engines but prefer 20% oil in my bushing engines. And all my engines are super happy.

My only fourstroke to date does its job in an helicopter, for little over 10 years now, and it is running on 10/10 full synthetic.
Changed the crankshaft bearings after the first season, and never again until now, adjusted the valves maybe 3 or 4 times in all those years.

Would not want to use that mix in my 2 strokes though....

Brgds, Bert

I use 10% Nitro Coolpower in all my glow engines

I use 10% nitro, 20% castor in ALL my glow engines. 2 or 4 cycle. Have been since I bought my first 4 cycle in 1976. Prior to that I ran K&B 100 in all my glow engines. It was 5 % nitro and 20 % castor.