how much heat/weld do you really think you'd be putting into the axles to light the oil???
seriously your more likely to bend the axle the amout your talking.
ive welded alot of brackets to axles using low hidrogen rods (lots of penetration and heat) an never had a problem, or even considered dropping the oil.
i presume you'd be using a mig, so at most you'll only be warming the inside of the tubes, you'd never get anywhere near the flash point of the oil or melt the seals which will be atleast 6 inches away.
by the way the axle tubes are about half inch think.