That V8 cap is shrouded, and offers even less room than the IL 6...
probably put on very tightly by someone, and the coolant crystallization
has "glued" the sumbitch on there...
Try a rag for better grip or, even a slip wrench, as pictured. Don't laugh,
I use my cheapazz one all the time on cars/m'cycles.
Be careful not to twist the coolant res. tank, tubes and unit: they are
all plastic and cheapazz parts.
And, left is loose, as in counterclockwise, as you noted.
GL,mD
strap wrench-Large Photo & Picture