I have always just scrubbed them with kitchen cleanser, like Comet or Ajax. Some of the thick spots of blue take a far amount of scrubbing, but always got the job done for me.
2014 Fusion 2.5 engine 5/20 QSUD
1965 Mustang 200 c/4 10w30 QS defy
1964 Ford Ranchero 302 c/4 15/40 Delo
1929 ford model A No filters, and 15/40 oil