my car cranks forever !!! any help??

[dsastr_clan]

i started to have problems with my 1990 240sx 2 days ago i just did an oil change to my 240 and replace the air filter with a k&n and since then my car has a lot of trouble when i tried to turn it on it cranks forever ! i have to give it at least 7 tries to the point that i even get afraid my battery will die then when it turns on it revs at really low rpm till it gets normal then it doesnt have any problems it drives nice and it idles perfect