This little O&R Gen-set manufactured April 13, 1972 belongs to a friend of mine and I promised I would try to get it running for him. Removed plate on bottom of fuel tank and cleaned out a 1/4" of thick gunk from the bottom of the fuel tank. All the brass nipples on the fuel tank and carb were corroded shut with green gook. Had to use tiny drills to clean them all out. Carb jet holes also plugged up. Replaced all fuel line, fuel clunk, air cleaner, diaphragm and a few other gaskets. Filed, cleaned and reset points gap. Cleaned rust on magnets and coil, reset coil to flywheel gap. Checked for spark nice purple blue spark. Ready to try and start the engine so I needed to mix some fuel and oil. I used some old Elf HTX 909 two cycle oil SAE 50. I had this oil from a few years back when I had one of those large RC cars that had a 2 cycle spark ignition engine in it. I mixed the oil & gas at 40:1 ratio with some regular ethanol gas I had and after a few pulls on the starter rope the genset came to life. Adjusted the mixter until the engine peaked out on sound then backed the needle off a quarter turn. Engine starts easy runs good and you could see a slight blue smoke coming out of the exhaust. Generator works great, puts out 115 volts AC or DC. My troubles started when I went to mix some new gas using STIHL Hp Ultra 2 cycle oil and 89 octane ethanol free gas. I mixed it at a 32:1 ratio like recommended on this forum for the O&R engines. After draining out the old fuel and putting in the new I started the O&R engine running for not even a minute the engine appeared to be leaning out RPM went down no blue smoke and the engine appeared to be getting hot. So I started adjusting the needle mixter open I was at 4 or 5 turns and the engine was still running lean. Tried turning needle in and the engine shut off. The new clunk I put in the fuel tank has a very fine filter built into it so I don't think any trash got in to the carb that might cause it to run lean. I tried my best but could not keep the engine running with this fuel oil mix. Drained the fuel out and put the old Elf HTX oil & ethanol gas back in it and with in a few minutes the O&R engine was starting & running good again. So can someone one tell me whats going on? The STIHL HP Ultra 2 cycle oil is synthetic. Does it not work with the old style piston ringed engines? I should state that I measured the 2 cycle oil using a MixMizer that has the different oil mix ratios on it. When you fill the MixMizer up to the 32:1 line this is for one quart of gas. So to mix one gallon of fuel you have to do this four times to have the correct amount of oil to gas for a 32 to 1 mix. I also used the same MixMizer to mix the Elf HTX oil at 40:1 so I'm fairly sure there was no mistake with the oil mix of either mixes. Thinking about trying some regular 30 wt STIHL 2 cycle oil mix. Any suggestions on what going on?