Most additives are garbage. A lot leave a little bit of an aftertaste and can make the oil more 'slimy.'
You should filter at least once a day. Even if you have to manually do it, it's still better to invest a little time every day, then rather then having to replace a unit five years earlier then you normally would.
For general maintence through out the day, A crumb scoop works fairly well, but you'll definitly need a fine mesh skimmer, as well as a clean out rod. I'm a huge fan of http://www.wasserstrom.com/restaurant-supplies-equipment/Product_120252 that one. It holds up fairly well to 350 degree plus tempetures, and is fine enough to get most of the particles. Skim it after every basket, and you'll see a cleaner oil.
We have a pretty wide range of fried items, and some things will destroy oil far faster then others. For example, Chicken karaage kills our oil far faster then chips, so our omnivore fryer needs changing faster then our vegan fryer. When we do a lot of karaage, we probably get 3 days out of our oil, whereas normally we get a little less then a week. We usually get about 7-10 days out of our vegan fryer.
Honestly, we could push the oil for longer, but quality>quantity. I'd rather spend the extra $30 on oil.
Oh, also, I'm not sure what type of fat you're using, but we use Expeller pressed canola oil, which lasts longer then chemically pressed. It also tastes a little better, in my opinion.