Oil will readily coat gold. This is the mechanics of a flotation process because the oil will make the extremely fine gold (<200 mesh) float. If you have oil in your process you may lose some very fine gold because it will simply float out in the wash.
Keep in mind that one small flake is equal to thousands of specks and also it is impossible to catch it all, there will always be losses.
In my experience I have found that increased production volume at the expense of losing a little fines will give you far more gold in your vial at the end of the day.
Dont worry about it...shovel on