What a general question with a million answers. I can tell you what the manual says, or I can tell you what I do.
The manual calls for a 10w40, rated SG or higher and the OEM Kawasaki filter.
I personally use Royal Purple, it is a synthetic oil that costs almost 7 bucks a quart, in 15w40, coupled with a Purolator PureONE PL14612 oil filter that I found at Advance Auto Parts for 5 bucks.
Works great, for synthetic oils, there are cheaper ones, like Mobil 1 gold cap at 4 bucks a quart, that is up to you though.
Just make sure you look at the back of the bottle of whatever 10w40 or higher you get and make sure it doesn't say "Energy Conserving," that stuff has a lot of slippery metals in it like molybdenum and zinc to reduce friction, and "conserve energy." It's great for an engine, but you have to remember your tranny gets lubed with engine oil. You don't want that in your clutch plates.
Anyways, dudes will say that you have to use motorcycle oil, which in most cases comes from the same batch as the rest with a motorcycle sticker slapped on it.
This topic has been beat to death, you can search for it.
Amsoil is generally considered the best, although I run Mobil MX4T as well. I also use a Scott's stainless steel filter, which is $120, but I'll never have to buy another one so it is a wash. Basically, if you do the standard maintenance, even basic oil won't break down in 3k miles. A lot of it is peace of mind; however, the synthetics do offer superior heat, rust and wear protection.