I don't wanna be a killjoy here, but the r/c glow plug is entirely pointless. That little plug will not make any significant addition to the available heat. Warming the engine up to operating temp on gasoline will provide much more useable heat.
RC glow engines run on methanol, usually with nitromethane added but not always. The glow plug works because of a catalytic reaction between the methanol and the platinum filament in the plug. Incidentally, it's a glow engine, not a nitro engine. "Nitro" is a catchy word for the new generation of people who want everything to be more exciting.
The glow plug is preheated by a 1.5 volt battery, and it keeps glowing because of the heat from the burning fuel. The catalytic reaction between the alcohol and the platinum doesn't happen if the plug isn't hot enough, so as long as the plug keeps glowing the engine will keep running.
Maybe the glow plug would help with an engine running on oil, but it wouldn't have to be a platinum plug, and it's not clear what heat range would be optimal for each individual fuel. Years ago I used to see an ad in the back of a magazine for a gizmo that you could use to turn an engine into a diesel engine. I'm pretty sure the guy selling it called it "The Gizmo". If I remember correctly, it was a screw-in plug for the model airplane market, to turn a glow engine into a diesel. In principle this would be a glow plug to hold just the right level of heat the same way a platinum plug does, but the level of heat would be different because the Gizmo was made of different metals and intended for a different fuel.
It seems to me that this principle should work for any engine that fires because of compression heat, in other words a diesel type engine. RC glow engines are technically called semi-diesel engines because the fuel is ignited due to the heat of compression, with the assistance of a small piece of metal that's hotter than the rest of the engine. A diesel tractor engine runs just because the air gets squished and it heats up enough to light the fuel. In any engine that's not capable of heating enough, or not capable of holding the heat with its lightweight aluminum casting, such as a small Briggs or Honda, it seems like you could add a small piece of metal designed to hold just enough heat to cause ignition when combined with the heat of compression. Maybe it could be a plug made mostly of copper or some other common metal, but you'd want it be capable of absorbing and holding heat locally.
Here's another thing you might want to consider, which may help you keep from reinventing the wheel. In addition to glow engines, RC airplane pilots also have the choice of using a true diesel. RC diesel engines use a fuel made of kerosene, castor oil, and ether, at a proportion of roughly 1/3 each. Kerosene itself is basically the same as common diesel fuel. The small engines require more lube, hence the castor oil. Also, they are not capable of holding enough heat to ignite the fuel due to compression alone, hence the ether. The ether lowers the flash point. These engines always have a contra-piston, which is attached to a screw on top of the engine head. To adjust the timing, you turn the screw, which moves the contra-piston, which changes the compression. This is done to allow the pilot to choose smaller or larger propellers to match the particular airplane the engine happens to be mounted on. If you tighten the screw, you increase compression, which advances the timing.
This leads me to the next interesting thing I've learned from RC engines, which is that there are head kits available to convert a glow or gas engine to diesel, but in a lot of cases the diesel fuel will pound the living crap out of the crank pin and break it off.
Also, your engine is equipped with a spark plug, which determines timing as well as initiating ignition. If you have this, the glow plug and heat retention qualities of the engine seem to be a moot point.
What does all this mean? I don't know. You can run just about any engine on any fuel you want, but there are always fine details that make it work better or worse. If your only goal is to run an engine on used fry oil, keep tweaking it and you'll probably get it to work right eventually.
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