Has been asked a few times but I would first try spraying with a QD and scrubbing with a soft brush, like a paint brush. If this isn't enough try some AG Engine & Machine Cleaner, leave that to soak on for a minute or two and try again.
After its clean rinse off well, and you can use something like Autosol or AG Metal Polish or AG SRP and then a wax or sealant to restore and protect the shine.