Its good to realise that mineral based chain oil isn't too good either for the environment or yourself. It takes forever to degrade and is mildly carcinogenic. As most of it would end up in the saw dust on the ground, you wouldn't spray a gallon of engine oil round your wood, so the effect must be similar.
A criticism of using straight vegetable oil such as rapeseed is that it can oxidise and leave deposits on the saw. My own experience bears this out, I used to use straight rape seed in my electric saw, and after a while it gets very sticky and you get a sort of resinous build up that has to be almost chiseled off- a bit like a frying pan that's never washed. Presumably the Stihl Bio oil is filtered and treated to stop this happening, pros who use it all the time don't report any issues. As its designed to break down very quickly, you wouldn't want to buy several years worth at once in case it starts to do just this- I'd have thought a gallon at a time would be optimum unless you're a pro. It'll set you back about £20 for the Stihl one, versus around £13 for a mineral oil.
I like the Arbtalk guy who said he had to stop using Bio as his dog kept eating all the sawdust and throwing up. It can make you hungry as it smells like chips.
