If it keeps fouling plugs the way you are using it, a step up in heat range won't hurt. You really need to get into the habit of running it 1/2 hour after it reaches operating temp every time you start it. Right now you are really rushing it towards deuteration. Short runs after a start up for a motor that sits for long periods is actually just filling the motor and exhaust system with combustion acids that need to be burned off every time after a start up. Those combustion acids will eat away at everything they are allowed to sit in.

If it ever gets to the point its actually used again (or run it long enough to burn the combustion acids out of it), you may need to drop back to the colder plug heat range. Hopefully when you start it up, you are at least able to put it in gear while its on jack stands, and let everything in the drive train get to move. Gene