if the glowplug is broken it will possibly ground itself and cause the wire to melt, best thing to do is to replace the glowplug.
also check the gauge of the wire to the glowplug, it should be quite large to handle the current.
with the oil, is it an oil bath air filter you are talking about?
if it is then perhaps your timing is out and some compression is coming back up the air intake.
if you are talking about the dipstick tube (or rocker cover holes), then you could have blowby in the pistons, which is a whole other kettle of fish to fix....
cheers,
Stephen