Mobil vtwin 20w-50 and Harleys own synthetic 20W-50 are certified for use in the primary chain case. Most EVO engines spec formula+ in the chain case which I believe a virgin analysis showed it to be a 30 weight oil. Formula+ is only about $8 or so. I would just use that once a year. Of note, on Sportsters, the chain case shares a sump with the transmission.