Two stroke engines
Two stroke engines do not separate the oil and the gas. The oil is mixed with the gas. These engines are generally used on chainsaws, weed trimmers, and other tools that are used at many angles. Chainsaws are a great example due to the various angles required for the tool to run when cutting down a tree. Having the oil mixed with the gas keeps the engine lubricated even when the tool is tilted or angled.
Tip: Use only two stroke engine oil in two stroke engines. Do not use car engine oil like SAE 10W-30W, or the like. Two stroke engines are design to burn oil which requires the proper oil in the gasoline..
Four stroke engines
Four stroke engines have a separate reservoir for the oil. They are often found on tools such as lawnmowers which stay horizontal most of the time they are in service. The oil stays in the bottom of the crankcase where slingers or other mechanical parts distribute the oil to the areas of the engine that require it. Another easy way to know is, generally, two stroke engines are on tools you must lift and carry, i.e., chainsaws and weed trimmers. They have many fewer parts than a four stroke engine so they are lighter. Four stroke engines can be found on tillers and most lawnmowers.