I bought one across from a gas station and prior to getting the Phase 1 in talking with the owner of the Environmental Services company he said he was confident based on the grade of the land, there would be no issue (because any spill/leakage from the gas station would head in a different direction - away from my building). 

This leakage is zero percent the fault of tenant. 

If the town confirms there is leakage, you have to run the tests and do remediation.

The situation:In-ground oil tank within a basement crawl space, which has been given a clean bill of no leakage by a third party and is in a Category A flood zone.

What HVAC installers have informed me is that keep A\C filter clean, use spray cleaner for coils when dirty, when most older A\C s were installed in Florida we didn't have the code requiring the retaping of vents with a quality tape to prevent leakage into the attic.Additionally the insulation, windows and doors are going to be contributing factors.You will be surprised how many people in South Florida do Not realize the amount of money being wasted by the Duct tape drying out and air venting into Non A\C spaces.

If I did not bring my management "in house,"  I would have significant financial leakage. 

You were purchasing a property with a leakage that was disclosed on the appraisal.

Again, it's dependent on a bunch of factors, like the efficiency of your furnace, leakage/insulation in the rest of the house, how often your duplex neighbor heats their side (if it has separate systems), etc.

If not, you probably want to do some soil testings just to make sure there was no leakage. 1970 is just about the time period when this was more of a problem, and I fully agree with @Philip Tretola, I would always heir on the side of caution when the risk is high and the cost is minimal comparatively.

If billed back, there is likely some leakage.