The "double wrench" means you need service (check the manual) and the -20 means you're 20 miles past the service time; a double wrench means a more significant service interval (Schedule B) is due.
Maintenance of Oil displays on the Dash
#1 Checking Mileage left on the FSS PDF
#2 Checking Mileage left on the FSS PDF
Checking the Oil level via the dash display PDF
(Located in the Index of Crossfire problems and repairs)
Posted below is the Double Wrench Service