[bonduranth]

Here is my best try at explaining plugs. The 8es is a cooler plug. The 4es is a hotter plug. The heat is a factor of the length of the inner post and the ceramic that surrounds it. The correct plug per factory manual is

25523-G3 SPARK PLUG (NGK BPR4ES -RESISTOR)........2

you should run the correct plug gapped at .022. After a few hours, you should pull the plugs and inspect them. There are several conditions that can be diagnosed from the color and coating (if any) on the plug tip. The introduction of ethanol in fuel has caused many headaches for those of us in the repair industry. It takes more spark to ignite the added ethanol, but when it does ignite, it burns hotter. You can go one number down to a BPR5ES if the plug show signs of detonation (spark knock) The tip insulator will be white on one side and have residue on the other side. There is no worry on the length, since the piston has plenty of internal clearance.

cross reference for BPR4ES

Accel: 114
AC Delco: R44XLS
Autolite: 4265
Autolite: 65
Beck Arnley: 176-9020
Bosch: WR7DC
Bosch: WR8DC
Champion: RN12YC
Champion: RN14YC
Denso: W14EPR-U
Denso: W14EXR-U
Mighty: M4GR52
Mighty: M4GR42
Mopar: 4091713
Mopar: 5213693
Mopar: 82300317
Motorcraft: AGS32C
Motorcraft: AGS42C
Napa: X761
Prestolite: 14G52
Splitfire: SM006E
Valley Forge: 34R