In a 1990 Ford Aerostar :
You either have the 3.0 liter V6 engine or the 4.0 liter V6 engine from the factory
( each size of engine has 6 engine cylinders )
The 1990 Ford Aerostar Maintenance Information manual says 21 gallons.
( there are 6 engine cylinders ) for the 1994 Ford Aerostar 3.0 liter V6 engine
The Ford Aerostar is a van that was produced from 1986 to 1997 by the Ford Motor Company. They produced 2,029,577 Aerostar vans in that time. This model was replaced by the Ford Windstar.
Ford stopped making the Aerostar in 1997
According to the 1996 Ford Aerostar Owner Guide : ( 21.0 U.S. gallons )
The V8 engine has ( 8 engine cylinders )
8 Ford or Chevy