I use Canola oil. That's what every kitchen I worked in while I was in the industry used, and generally to my knowledge it's what most restaurants today are using. You can pick it up at any grocery store. I get the 5L bottle that Walmart carries. (it's about $20 after taxes)
How many times before disposing of depends on how often you're using the fryer, what you've cooked in it, and whether or not you skim the top after each use for any breading that has come off whatever you were cooking. If you don't, it eventually settles to the bottom of the fryer, burns and that will change the life expectancy of the oil. I just changed the oil in my fryer yesterday. In general, I change it every 4-6 weeks depending on how often I use it. In the restaurants, because they were used so heavily, we were changing the oil at least once a week, sometimes twice when we had used them to prep things like fish for fish and chips, or chicken balls, deep fried shrimp, egg rolls etc in the N.American/Chinese food restaurant I worked at.
In the restaurants I worked in there were usually 2 fryers, (sometimes 4 and when I worked in the restaurant that did both North American food and Chinese it had 6 fryers.) One was for fish and seafood ONLY. One was for fries only so that there would be no "cross contamination" as people with seafood allergies for example, could become very ill (possibly causing death depending on the severity of their allergy) if you fried their fries or onion rings in the same oil you fried the fish and seafood in.
That's why when I do fish either battered or breaded where I'm doing it and it hasn't been store bought already breaded or battered, I don't use the deep fryer itself. I put older used oil that is still usable in a deep pan on the stove and do it there instead of using the fryer to lengthen the life of the oil in the fryer, (and save myself the money and time of having to change it more often if I didn't do it that way.)
When the oil has run it's life, (you'll know by the color both of the oil, and the food you've cooked in it, or if it smells burnt) I put it into the container from which it came, put it in the freezer, and on garbage day, it goes out with the garbage. (Green bin now) The oil I used yesterday to do that fish for example could be used one more time. It was the old oil out of the fryer after I changed it, but being garbage day is tomorrow, it's going out with the garbage tomorrow since I won't have to do fish again for a while, and by the time I do decide to do fish again, the oil in the fryer will likely need to be changed again.