The only lubrication that a two stroke engine has, is the oil that is in the two stroke mix. The less oil that is in the gas the less lubrication your engine has. If your fuel mixture is too light you run the risk of seizing the engine or spinning a bearing. It is the same as if you were to run your car engine a quart low on oil. The internal combustion parts will not be adequately lubricated and the result will be excessive wear on the engine.
The Stihl fuel mix ratio for all Stihl two stroke engines is 50:1. That is 50 parts gas to 1 part two stroke oil mix. This is the oil ratio that is recommended by the manufacturer. You may however use a thicker fuel mixture like 40:1 or 30:1, just be ready to change your spark plug more frequently. You will notice if you run a heavier gas to oil ratio the engine will smoke a bit. Don't be concerned it is simply the engine burning off the excess oil.
I frequently run my Stihl chainsaw slightly heavier than 50:1. I use a 2.5 gallon size oil mix and mix it with 2.4 gallons of fuel. I have run many Stihl chainsaws with this mix ratio for many years without any trouble at all and no noticeable smoking from the engine. It makes me feel better to have that extra bit of lubrication in my engine.
Older Stihl chainsaws may require a thicker mix. If you have an older saw and I mean 20 years old or older it may require a mix closer to 30:1. If you are not sure it would be best to use a thicker oil to gas ratio. At worst you will have to change the plug more often.