Kinda reviving/creating a new thread on Harley-Davidson primary oil. The latest thread dates back last year and , no pun intended, primarily deals with using ATF in the primary. I'm due for a primary change and currently have Rotella 15w40 dino in it. On hand to avoid going to the store, I have lots of M1 15w50, M1 4T 10w40, M1 V-twin 20w50 and Mystik JT8 15w50 syn blend. Looking at M1's website, back in 2007 (!!) they recommended the 4T 10w40 over the V-Twin 20w50. Current bottles of the VT 20w50 show that it's good for all three holes. I called M1 tech support and they were stumped!! The tech stated he was "sure" both would be good, but thought the MFR's viscosity would be best. I said that goes against what M1 has on line. The tech stated he would research/dig around, and get back to me. Oh well ... I'm sure any of what I have would/will be fine, but what are some H-D owners thoughts ?