ORIGINAL: jmpups
I can't tell you that Marvel Mystery oil is good or bad to use as after run. The one thing I can tell you is that my first and last experience with this product was about 10 years ago when I thought I was doing A good thing with A OS108 motor that I wasn't going to use for a while. I fully lubricated the intake and exhaust ports with the Marvel oil, then put it in a plastic vacuum sealed bag. A year later when I opened the seal to use the motor I found a rusted solid frozen piston, the engine was completely seized up and useless. For what it's worth. jmpups
I have a feeling you were using more than 0-5% nitro and didn't allow the crankcase to vent sufficiently. The result is the marvel oil basically coated the parts locking the moisture and any nitric acid residue in. A guy I know had the same exact thing happen with a .25FX. I don't recall what oil he used exactly, but I believe it was an automotive mineral oil. Filled the engine completely with oil, dumped it out and stored it. Bearings rusted solid.
I suggest venting the crankcase of 2-strokes for a few days (piston at BDC) before loading with oil. This is my practice, though I use 5% nitro in all of my engines except one. I rarely use anything for storage in the 2-strokes as the castor does a good job of keeping stuff rust free. My Enya 60-4C got filled and stored full of ATF for the winter. Turns smooth as butter