I'm clearing land and most of the trees are being chipped for mulch. But some of the brush and vines are just too labor intensive to get them in the chipper. Also, As i pull out roots and stumps, they are too sandy to put thru the chipper lest I ruin my knives. So I'm creating several burn piles. Does any one have any data on using used motor oil to light the fires? Any significant danger of chemical leaching into the soil?