Done about 1,000 miles with a K&N fitted to my XT600E. It was fitted by the previous owner, when he changed the exhaust too. No problems at all, but I haven't checked it yet.
K&N oil is probably the best, but I should think any the same weight will do the job. A big selling point of the K&N is that it is washable.
The important thing is that the mixture is checked when changing an air filter, as if it's less restrictive, (which they are, compared to std) more air flows in leaning the mixture off. If it was more restrictive, less air mixes in, giving a richer mix. Adjustments may have to be made to set it up right.