The Shimano theory of oil dip is to regenerate the grease, which can dry out. It is NOT to run in an oil bath (such as the SA, Rohloff and Alfine 11 hubs).
I stripped my 3 year-old Alfine 8 after lots of hard, all-weather use and about 1400 miles. The grease was still good but I dipped it and added grease to the non drive-side (adjustable) bearings.
It is noticable more efficient and runs more freely.
I used some cheap brand automatic transmission fluid and drained it for 10mins.