Erik
The one time I saw K&N oil, it appeared to be equal to a heavy engine oil - 50 wt, or an 80-90 wt. Gearlube, which is about equal thickness to a 50 wt.
ATF is almost always a 20 wt. so might be too light... but, if it worked before, might be ok.
Best way I know to tell is to monitor your intake hose to the turbo. If it's clean your probably ok.
JMO but overoiling has always been the bigger problem.
That being said, it's only @ 40,000 miles or so. . why take a chance NOT using K&N oil... it may be some special formula and the per mile expense is peanuts.
RJ