I use silicon tubing with no problem.

Ditto on you may be adding too much oil. Make a restrictor, it's easy, you'll get better smoke, and the oil will last much longer.

I heard that lamp oil is good too, although I've never tried it. I use Ultrasmoke. I don't have the phone number with me right now, but I'll post it tomorrow. I't $10 a gallon, but it's worth every penny. It makes great smoke, and it's not petroleum based, so it has no smell and won't stink up your car (Or house), and you can use any type of tubing.