I've been running the same combination of oil in my 01 EV for 110,000 miles with no issues. The original boot is still in place. Try this: ride the bike long enough to get it well into operating temperature. Shut it off and remove the fill plug (the top one) on your rear drive. This will burp the expanded pressure from the unit. Re-install the full plug and tighten it. See if this helps solve the issue.