After solving my poor running problems and then my mysterious noise (replaced clutch and all is well), I took my first real drive in months yesterday. After about 3 miles, the engine began to lose a little power (like running on three, haven't pulled plugs or checked compression yet) and some serious blue exhuast smoke appeared. No unusual engine noises.

The only engine change since my last drive is that I added an 8 oz. bottle of zinc additve (sold for use as a break in lubricant). I had just changed oil and, since I had no Brad Penn on the shelf, figured that the additive would be a good substitute.
Any chance that it could have done something to cause serious oil burning? I know I may be dreaming, but all the other causes (broken ring, valve guide, etc.) are too unpleasant to contemplate.