I am selling my property (an auto repair shop) and the sale agreement requires a Phase I environment study or report. Exactly what do they look for? Should I clean up the surface oil spots before they come?

HI,
Thank you for asking a question here, and Welcome to the site.
I am not a professional environment person, but can give you some insight.
http://www.assessco.com/services-ph1.html
The above link explains all about the Phase 1 Environment Study, and the link is a consulting firm, who specializes in these types of studies.
I would definitely clean up any oil spills! But, after viewing the above link, you will see that this is not all the Study is involved with.
They will do some drilling of the soil surrounding your place, analyzing the soil. Anything found underneath the surface will have to be cleaned up.
For example, many in my local area have underground oil (diesel) tanks, used for older type oil furnaces for heat in the winter. Those tanks have to be inspected, if still in use, or removed if not in use, with any leakage into the soil cleaned up.
I do wish you the best, and good luck.