Chainsaw fuel is a mixture of petrol and oil. The oil is good quality two-stroke oil, suitable for chainsaws. The mix is 50:1, petrol: oil, so that’s 100ml of oil to every 5 litres of petrol. You can collect petrol from your local garage in a jerry can, but not the oil – it’s available from chainsaw dealers or agricultural merchants. You can buy it in 100ml sachets to add to 5 litres of petrol, or you can buy a can that will last for years, and measure and mix it yourself.
Don’t leave fuel in a chainsaw for more than a couple of weeks – they don’t like fuel sitting in the carburretor or tank for more than that. So if you’re chainsawing, and you know you won’t be using the saw for a while, keep going until it runs out of fuel and cuts out.
When you add fuel, you also need to add chain oil for lubrication (get it from a chainsaw merchant). There’s a separate tank, which empties at more or less the same rate as the fuel tank. When the chain is moving (i.e. not when it’s just ticking over), the lubricant pumps automatically. You must remember to add oil when you add fuel, otherwise you will damage the bar and/or chain very quickly.