The factory hardline is 3/8 OD so the 3/8 compression fittings to -6 works and is what we use. The -6 to 1/4" NPT fitting on the turbo isn't radiused so it acts as a restrictor anyways and is just like its a -4.
you can get a jic fitting from elite that goes into the oil cooler base and then just make a hose from there to the turbo. That way you don't have to worry about a compression fitting leaking.