Mineral Spirits will evaporate and leave the grease pretty much as it was. The real question besides how in the name of heaven did mineral spirits get spilled--is whether it wicked itself to the aperture blades or shutter mechanism. The best thing is to turn you oven to the lowest setting, turn it off and put the lens in to bake. Repeat the cycle all day, if necessary. In a day or 2 the solvent will be chased off and the original grease will be as it was. Quite possibly, no mater what you do, a greazy fog will cover the lens surfaces. But maybe not. Just don't let the oven temp get too high.