Assuming that your muck was not polluted some how, it should grow plant life well. My 1/4 acre pond was mucked out last year (8 foot deep smelly barnyard muck) and merely pushed over the dam. This spring the drying muck was smoothed over with no top soil covering. It is growing grass (rye and fescue) wonderfully. I used some of the dried muck in the yard in some low spots and the grass is better there than anywhere else in the yard. I don't have any wet spots however, nor have I seen any oiliness.

This is my only experience with muck so take it for what it's worth. I wonder if you plowed it up to help it dry, expose it to some light and air if that would not speed the process up. The only thing I can think of to explain the "oily" water (besides downright pollution)is that plants contain oils and it's possible that areas of your pond muck where made up of more decaying plant matter than other areas.

I can't recommend any additives, but I'd try tilling it up and possibly mixing in some regular soils after it has had a chance to dry out. Of course, leaving it be might prove out before the summer is over.

Fish on!,
Noel