Stock bore on the B18 is 3.313 inches, so your pistons are a bit undersize probably from wear. Any scuffing on the skirts? I haven't measured any for a while but new they would be about 3.312 or 3.311. The pistons are stock size, but you should measure the bores with a taper gauge or have it done, after honing. If there is any kind of ridge, like .003 inch or more, then you are pushing it for a ring job. If you've got .005 clearance between piston and bore at the top after honing you'll be fine. Maybe a thou or two more if you're not going to thrash the engine.
More deposits on the pistons is a good thing. The pistons that pass oil will show cleaner areas as the oil burns the deposits off. Normally shows up as a half moon shaped clean area.
Plugged oil return holes and weak oil ring tension causes oil burning.
The D is a factory marking used to identify select fit pistons to get the most accurate fit on assembly. Means nothing now.
I don't know where you can find rings, but cast iron ones are best in a taper, and three piece oil rings work really well.