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You are right that you should be using one tank of bar and chain oil to a tank of gas. Anything less is just not right. There are a few things that you can check or have checked. First we need to make sure that the oil pump is being driven. You will need to remove the bar cover and the bar and chain. On the sprocket you will see an e-clip, remove this and then lift off the clutch drum from the engine. When you have the clutch drum off you will see a little notch in it. This is what drives your oil pump when the clutch is turning it and the chain. Now look around the outside of the clutch with a flashlight, you should see a small hard wire poking out from underneath the clutch were this slot in the clutch drum rides. If you see it then it may just not have been lined up with the clutch but lets check the whole thing out anyway. You will now need to remove the clutch. First we need to stop the engine from turning over. Remove the spark plug and insert a length of skinny rope about the size of your pull cord rope. Now place a wrench or soc.et on the clutch and turn it off. The clutch has a reverse thread so to loosen it turn it to counter clockwise. Under this you will also see a large washer that can be lifted off. Now you will see the wire I was talking about. It should be attached to a plastic gear that will just pull off over the crank. You may need to turn it a little to get it to come off. The wire should be firmly attached to the gear and the teeth on the gear in god shape. The teeth are what turn the oil pump. Inspect all of this. If all of it looks good you will probably need to replace the oil pump.
If you need part numbers and or pricing for your parts just ask and I can forward them to you.
The plastic gear and the small hard wire come as an assembly. It is part number 1123(NNN) NNN-NNNNand should cost you $10.70. You may be able to find this online somewhere, but Stihl is very strict about parts being sold online or outside of dealerships. Go to Stihldealer.net and input your zip code and the closest dealers to you will come up.
You are probably all set with the oil pump. If you want to replace it you will have to bring it to a shop These require a special tool to remove and are pressed into the saw housing in just a certain way and can be a pain in the neck to change