In budgeting time and money for a house I just purchased, the inspector noted that the central air conditioner is mounted outside without a pad underneath. The unit is a Carrier 38CKC024330, manufactured in 1999. The product data I found online lists using a unit riser (a.k.a. pad) as 'Recommended.'
I live in the Midwestern United States, which enjoys high humidity and harsh winters. I'm not sure how good the drainage is in the area of the unit, but it is mostly sheltered by the eave of the roof.
Given the age of the unit -- both how it may be nearing end of life, and how it has survived this long like this -- is it worth installing a pad underneath? Are there alternative techniques, say digging out a perimeter around the unit and filling it with gravel?