First let me say that you did fine using anti seize, plugs can and do seize in the head under certain conditions all of the variables are too many to post.
I have used it on almost every plug i have ever installed in every engine since 1972 and never had a problem, back in those days with 10K tune ups in a dealership environment as an apprentice that could have been 40 plugs and 10 sets of points on a busy day.
One engine stands out in my mind, the Ford Kent engines. If you didn't use anti seize on these and left the plugs 30K you would (not maybe) end up pulling the head to drill them out.
The real issue is not so much a performance issue or a good or bad issue as it is a liability issue, although these can be all problem areas if the product is misused.
Most manufacturers have a published torque spec even in their owners manuals and when anti seize is used that figure must be reduced or stripped thread are a real possibility especially on aluminum heads.
Realizing this is one of the biggest DIY performed by car owners it would be amiss for a company like NGK to advocate the use of this product.
Doing so would mean they have to take it upon themselves to amend the manufacturers spec and assume the liability for doing so.
No company is going to open that door so they coat the thread with zinc or nickel which should prevent seizing, it doesn't change the torque value but it isn't ideal either.
The other issue is they have no control of the application of the product. If too much is used it can short the plug out causing a misfire and possibly cat damage.
Over tightening can lead to stripped threads or seat galling, why would they even dream of recommending this product?
Proper application is an absolute must just a light coating on the threads no where near the electrodes and a reduction in torque by 30% is a must unless the manufacturer specs for wet threads.
Eg Honda on some engines years ago recommended engine oil be used on the threads and their torque spec reflected this.
Ford was having some problems with stuck threads and immediately recommended anti seize be used.
As far as Champion not recommending it, they offer a anti seize just for park plugs.
http://www.aircraftspruce.com/catalog/eppages/antiseize.php
Notice it is for aircraft. Not much DIY but done by a professional mechanic.
I have always lived in a salt and high corrosion area. In other areas where corrosion is not an issue you probably don't need anti seize on anything.
This little rant is based on my experiences and opinions gathered over decades of working in this kind of climate.
Edit: As far as oil on the threads, just look at the boots, if its wet with oil then as others have said tube seals.
Not a 30 min job on this engine.
