Swapping bars on my Poulan WildThing

Oregon has a chain/bar selector guide here:

http://www.oregonchain.com/selguide.htm

The bar on the saw is probably made by Oregon, but it really doesn't matter.

The original 18" bar just needs a loop of 62 links of 3/8" low profile (link length) .050 (bar groove gauge) chain. It can be Carlton, Woodsman Pro, Oregon, Laser, Husqvarna, Stihl, or any other brand chain. It just needs to be 62 link 3/8LP .050 chain.

If you want a shorter or wider bar you'll need a different size chain.

Note that 3/8LP has shorter narrower links than true 3/8" chain. Real 3/8" (.375) as well as .325 or .404 chain will not work with your current bar and drive sprocket.

KarlP is right on the money.

Just in case you don't know it, you should periodically flip your bar around so that it wears evenly. It will last longer and cut better if you do. Don't forget to clean out the oil port on the side of the bar, because the side not being used always clogs up with crap. I would recommend flipping the bar every 10 tanks or so, depending on how hard you use your saw.

Wah,
if the bar is in good shape it would be a waste of your hard earned money to replace it. Oregon has an awesome website with all kinds of chain & bar application info. You can mix & match bar & chain brands as long as they are the right ones for your saw & are the correct for the bar length. I'd also stick with the 18" bar, unless the bar is worn out. Then you could switch to 16" bar & chain if you want. If you swing around in trees often, it is easier & safer to have a shorter bar. Chain & bar life can be maximized if you remove the bar every 2nd tank of fuel & REALLY CLEAN IT! I ground an old screwdriver down so it fights kinda tight in the bar groove. With the bar off run it down the chain groove & remove all of the fine sawdust & bar oil schmaag. Then, blow the groove & oil passages out with compressed air (or clean with a wire like a pipe cleaner). Do this religously every second tank of fuel & you will be amazed how ling the bar & chain lasts, only like you stated, If you keep the saw out of the dirt. Keep the chain tensioned properly. Also, never let your saw run out of bar oil. A dry chain will lunch itself and the bar in a very short time! I keep my spare chains in a coffee can soaking in diesel fuel. The fuel removes dirt & dissolves any wood pithch that accumulates on the chain.