Hydraulic fluid is an petroleum, mineral or in some rare cases vegetable based oil that is thermally stabil, so it have a very high evaporation point. It does not normally give off hazardous vapors unless in enclosed spaces or high temperatures. An example of when you might have vapors is if the oil leaks onto the engine manifold. In that case, it smells like most oil or petroleum products when they burn.
I have a number of forestry clients that are starting to use the vegetable based oils simply because of the high cost of cleanup in the event of a spill, especially to water.
Shingles are manufactured from modified asphalt on an organic or fiberglass substrate. Granules are applied while the asphalt is hot. Oil and grease contaminants can cause premature roof system damage. I see this at some restaurants and industry locations with oil mists. It can cause the roof materials to discolor, soften, delaminate, blister or crack. I would expect to see the effects from a one-time oil contamination fairly quickly, and would consist of a change in appearance, hardness, or loss of granules. IF damage ocurred, this should be something you can actually see or touch. If the affected area, after cleaning, does not appear or feel different from the rest of the roof, I would be surprised if anything would turn up later.
A release of hydraulic fluid can cause property or environmental damage. The spiller is liable for the cleanup and proper disposal of oil wastes. My suggestion is that you simply write a letter to the company documenting the time and specific location of the accident. Thank them for the cleanup, but inform them you will expect reimbursement or repair if the affected area fails prematurely. Keep a copy for your records. Thats about all you can do. If nothing shows up for the rest of the summer, you (and they) are probably home free.
Is there any remaining discoloration or texture change?