Are you sure it's fuel?
Gasoline tends to evaporate very quickly and leave no trace. A splash of gas on the filter will dry up and disappear in easily within a minute or two. Unless the filter is actually soaked in gasoline which readily dries up in a few minutes, I do not think you have a problem, other than a possible defective PCV valve.
If you find the filter looks stained and smells somewhat like gas/oil, you are probably seeing the result of crankcase gases that have been ducted into the aircleaner. This is normal in older, higher mileage engines.
Any engine will produce a fog of oil vapor and partially burned hydrocarbons (blow-by gases) in the crankcase when running. Older, more worn cars like yours produce more gases than new cars.
A measured amount of these gases are sucked into the intake manifold through the PCV valve to be burned in the engine. This reduces pollution and prolongs engine and oil life.
When the engine produces more gases than can be sucked in (common in old engines), the excess gases are ducted into the air cleaner.
The gases typically stain the airfilter and make it smell of gasoline and oil.
This is normal up to a point. If it's excessive, your PCV valve is stuck or blocked and needs to be replaced. They are very inexpensive and easy to change. If you change it, make sure the tube itself is clean inside.