Jim, this is a two stroke engine. You must mix the fuel and the oil together. Typical ratios are 30:1 - 50:1. Most saws nowadays run at 40:1. Using oil from years ago, 2 strokes used to run at 30:1. The oils are more effective now and thus less can be used. Your fuel tank holds this mixture of gas and oil. If you use straight gas, you will burn up the saw, the engine will run hot and without oil. The cylinder will get scored. The hard coating on the cylinder will probably scrape off. Eventually the piston will seize. You likely will not be able to recover the cylinder as it will have scoring preventing a good seal (compression) and your piston won't move smoothly. You can hone the cylinder but it would probably not be worth it. The rear tank(near the handle) is the fuel tank. The front tank, near the bar is your bar oil tank. This oil only gets distributed to the bar. If you run without bar oil, you will quickly dull your chain. You will also heat up your chain and bar. You could eventually ruin your bar and chain by overheating, but you will know fairly quickly that your saw is not cutting well. You will see smoke coming from your cut and even burn marks. The chain may get really loose as it expands. The bar and chain will be too hot to touch with your bare hand. If it is too hot, you need to sharpen, make sure your chain is on the right direction and/or make sure your chain is getting oil.

Your saw might fetch $150.00 in good condition here, I don't know what prices are like in your area. The 450, 270 and 280 are not pro quality saws, they are designed for the occasional user/weekend warrior. What length bar do you want? The MS 260 would be a good choice but is limited to 20" and .325 chain. 3/8 can go on with an Oregon bar but the saw doesn't have the cajones to pull it. What about a Husky 359, a lot lighter than your current one but uses the same 3/8 chain. Good all around saw.

Okay, time for me to come clean...I talked about this about 2 years ago. I've done it, but it is dangerous and there are better, cheaper and faster ways to do it. Some sagely advisors from this forum tried to talk me out of it, but I didn't listen. 1. It will work...for about 30 seconds or so. It is the same thing as trying to use a chain saw to cut through rootballs of trees (which also have soil/dirt/rocks in/on them). Even a small, trace amount of dirt is enough to dull the blade. 2. Blades aren't cheap...18-28 bucks at my local HD. You are gonna go through ALOT. Dull blades are NO FUN to sharpen...takes for ever. Changing blades gets boring. 3. You are gonna burn up the saw eventually. (I burned through two of em). 4. It just ain't safe. Ever see a chainsaw kick back off a rock? Ain't pretty. Now that I am older, and perhaps wiser, I've opted for 12 inch sawzall blades. You can buy a pack of 5 at my local HD for 16 bucks or so. There are also 'slammer' shovels that I hear are very useful. I've also used a 40 lb. jackhammer with a modified clay spade (made it a reverse "v" and sharpened). Works well too, but you have to sharpen the spade every so often. but found that it was just easier to use the sawzall.

Apparently the saw is set too rich , which was the norm back in the 60,s vintage saws. Also these saws had wrap around fuel tanks close to the cylinder heads which cause fuel to boil often once saw was shut off for a period of time. This would cause very poor restarts . Needing you to let saw cool off prior to restarting . Advice never shut saw off till your finished cutting . Or run saw till empty and allow to cool and refill then start . Sorry no happy medium with that model of older Mac .