For god sake do not run it on neat fuel, all modern chainsaws use two stroke oil and would generally use a 30:1 - 40:1 mix just to make sure - seized engine verses a little smoke - know which one I would preffer.
Bar oil can be bought from screwfix.com for £4 a litre or just use the Stihl oil - 5 litres circa £12 - only use alternatives as an emergency option
Steve, May 2009