The armor-all trick really works, but it does have 1 side effect. Armor-all contains silicon, which will melt itself on to your glowplug. Over time, this can result in plugs that don't work right. They will slow with the ignitor on them, but the element is covered, preventing the fuel from coming in to contact with it. The more Armor-all, the faster the problem occurs. Just a couple of drops in a gallon should be all you need, and with any luck you'll blow the plug for other reasons well before this buildup is a problem. But if the engine starts running funky after you've put a lot of treated fuel through it, try the plug.