Stihl grass trimmers require a pre-mixed form of petrol and oil to correctly lubricate the piston and the crankshaft. Unlike other engines, like automotive, these two-cycle engines don't have an oil pump to do the lubricating. So this lubrication comes from your fuel mix. This mixture, to deliver the proper lubrication, must be mixed precisely according to the manufacturer.
The type of oil needed for this two-cycle engine is an air-cooled engine oil. This means that any oil designed for water-cooled or other kinds of cooling systems will not work in the engine. For example, boat engines, outboard engines and snowmobile engines use water-cooled and air-cooled engines, which, although they can be two-cycle, are far different than your grass trimmer's engine. Use only the highest quality air-cooled engine oil to prevent excessive heat and friction inside the piston. Stihl tests all of its engines with a Stihl engine oil. Although Stihl oil isn't a requirement, it could help improve engine performance and extend the life of your trimmer.
Stihl recommends mixing your oil with a grade of petrol at 89-octane or above. If you can't find 89-octane petrol in your area, use the premium octane gasoline. This petrol will reduce heat inside the piston and will reduce the risk of seizing up the piston. You must never use petrol with additives, such as ethanol, or other blended types of petrol. This will have a detrimental effect on certain plastics, like the carburettor's diaphragm, inside your engine. Use only unblended high-quality petrol from a nationally recognised brand.
The mix ratio for Stihl gas trimmers is on the higher side for two-cycle engines. At a mix of 50-to-1 petrol to oil, these engines require less lubrication than many other counterparts. This ratio was developed in the Stihl laboratories using the Stihl oil and petrol recommendations. Going above this mixture will greatly increase engine temperatures and risk of piston seizure. Going below this mixture may cause excessive carbon build-up in the muffler and exhaust port of the engine.
When filling the fuel canister at a filling station it's important to always remove the canister from the back of your vehicle. Static electricity can build up and spark, causing a dangerous fire hazard; always put the canister on the ground before adding the fuel. Pour the oil into the canister first and then add the petrol on top. When the petrol is mixed, close the container and shake it vigorously to mix up the liquids. Never mix more fuel than you can use in a period of about 30 days, which, at that time, will begin separating.