I'd suspect the valve cover gasket is failing and oil is seeping into the plug holes.
Mine has the sooty exhaust and hard-to-start after sitting, and I think it's valve seals. Mine has well over twice the mileage of yours though and mine smokes from the exhaust.
Yours probably just needs a good tune-up (60K is timing belt time anyways)!
Yeah, the valve cover gasket will cause the oil to come down from the top. The o-ring looking piece dries up and oil seeps past and can only go down into the plug hole since the coil blocks it from going up.
Have you checked the timing?
Fuel filter is good, so are fluids.
In reply to lizard:
In the diff I use mt90 and it takes just over on bottle (1L). In the tranny I use Mtl, but don't remember how much. Miata.net garage section will tell you.
Belt dressing never really works, and if you think about it a can of belt dressing costs a decent fraction of a new belt.
On the other hand it may just be that the alternator belt needs to be re-tightened for it to quiet down, mine was just like that on damp mornings too.
Belt dressing is only a temporary fix. Check the tension on the belts.
Just buy the factory gaskets from Med Center Mazda.
Check the tension on the belts, I bet they are too loose.
Skip the Chilton and buy Keith's book from Flyin' Miata. He even signed mine 
Absolutely invaluable to a Miata tinkerer.
Javelin wrote: Skip the Chilton and buy Keith's book from Flyin' Miata. He even signed mine
I would buy both. They really arent substitutes for each other. Keiths book is awesome for pics of complicated stuff. Mine is greasy and well beat up!
Joey
+1 on a manual. I had a Hayes one (instead of a Chilton). Well worth the $15 I spent on it.
Couple of tips for a VC gasket and cam seal change. Before pulling the CAS use a sharpie to mark it's position. The timing changes by rotating it. The line you mark on it will get you back to where you need to be without a timing light.
When tightening up the valve cover studs the reccommended torque is in INCH-pounds not foot pounds. Make sure you use the right torque wrench for this. Many a studs have been broken because somebody got all ham fisted with the wrong wrench.
Ignore all mentions of a CAS or o-ring. That applies to 1990-97 cars, not your 1999. You'll just go nuts looking for it and it's not there.
And yes, the valve cover gasket is why you have oil on the plugs. I'd definitely keep monitoring the oil. The slow starting is a 1999 feature.
Transmission takes 2 quarts, plus a splash for the shift turret. Use Redline 75/90 for the diff, MTL for the trans.
I think Keith's book is awesome 
My second favorite is the factory publication, but it's written with the expectation that you do have some clue as to what you're doing. One of the Chilton or Haynes manuals is a terrible thing, but I forget which!
Yeah, that Keith guy has three Miata books:
Miata Performance Projects: how-to. It's the one that's going to tell you how to change valve cover gaskets, shocks, sway bars, tops, etc. This is the one you want if you're planning on working on the car. For 1990-05 cars, although it was published before the Mazdaspeed turbo hit the market.
Find It, Fix It, Trick It: cheesy name. I like to think of it as "Miata 101". History, how to buy, year-by-year breakdown, information on popular upgrades. 1990-05.
How To Build a High Performance Mazda Miata: hitting bookshelves in the next month or so. This one's the theory. Instead of how to change sway bars, it covers what handling is, where it comes from (weight transfer, etc), the contributions of all the suspension bits, and how to adjust your various suspension bits to accomplish what you're trying to accomplish. Just about every aspect of Miata performance is covered. Covers 1990-10 models, although a lot of it applies to every car. A sample chapter is going online at flyinmiata.com later this week.
All available through your major booksellers such as Barnes & Noble or Amazon, but only Flyin' Miata has the autographed copies!
I wanna be just like you when I grow up, Keith. Not even joking.
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