I doubt that changing the brand of oil would have any bearing on the engine's performance. Oil brand/composition just isn't that critical to have a major effect on engine performance.
I'd suspect that you had some form of fuel restriction. Be it a piece of varnish that may have dissolved or a small piece of debris restricting a jet or passage dislodged, a kink in the fuel line, an air bubble, a fuel valve restriction, a fuel tank ventilation issue, a piece of debris between electrodes on the spark plug??? Any number of things could have had an effect but I doubt very much it would be brand of oil.
I understand that it's the only variable that you introduced. However, as mentioned above, it's more probable that it was caused any of the above possibilities or something else.
I have a small Shindaiwa two cycle water pump that was causing me issues just this weekend at camp. Sometimes it would run fine then not at all. An intermittent problem. Pulled the carb, cleaned it reassembled. No change. Pulled the fuel line and it would allow fuel flow. Reassembled, no change. Pulled fuel valve inspected, found no problem, reassembled, tested fuel valve, no fuel???? I thought that I had thoroughly inspected the fuel tank but, I soon found that what was happening. When I would work on the pump, I laid it on it's back to keep the fuel at the deep end of the tank opposite the fuel outlet. When I sat the pump upright to test it, the fuel would slosh back to the fuel line outlet. As a result, it would dislodge a very thin piece of debris approx 1/4" square (piece of a leaf possibly) that was stuck to the side of the outlet spout on the fuel tank. If it stayed stuck to the side of the spout, the pump would run. However, the sloshing fuel would sometimes cause it to drop and block the 1/8" diameter fuel line hole. Removal of the debris did the trick.
Little things like that can be very difficult to diagnose if every aspect of the fuel system isn't thoroughly examined.
