Welcome to the forum.
You have a 4.0l OHV engine in a 2000 Ranger, used from 1990 to 2000
2001 Rangers got the 4.0l SOHC engines.
You were given some bad info.
1990-1994 4.0l OHV pistons had a shallow depression and large head chamber.
1995-2000 4.0l OHV pistons had deeper depression and smaller heart shaped head chamber.
So both had same compression ratio
Your compression with '96 pistons and 2000 heads would be stock, 9.0:1, no need for premium gas.
'94 or earlier pistons with '95 or later heads can get about 10:1 which would need premium gas.
People prefer the '95-'97 heads for this because in '98 the exhaust ports were made smaller to increase the velocity of the exiting gases, the increased velocity heated up O2 sensors and Catalytic converters faster for better cold start emission numbers.
Have a read here about changes to the 4.0l OHV over its life: http://www.therangerstation.com/tech...0rebuild.shtml