I used to use Mobil 1 V-Twin 20W/50 in my Duece and my wife's Lowrider (both Twin Cams) but just switched to the new Redline HD 20W/60 because I got 2 cases as a gift. I had no issues with M1 and can recommend it. I'll let everybody know how the Redline does on my trip to Sturgis coming up. It seems real thick, maybe more thick than needed. As for the transmissions, I use M1 75W/90 gear oil and I use the Harley Formula + in the primarys. I feel that the harley primary/trans fluid is the best thing for a big twin primary, but the gear oils work best in the trannys. In any event I steer clear of the Harley SYN3. I did try the oil in all 3 holes and was so dissapointed in the rough shifting that I dumped the trans fluid within 200 miles. Once the trans fluid was changed back to M1 gear oil the tranny was fine again. (I did run the SYN3 in the motor and primary until change time).
So, my feelings are that the Harley oil is (like G-Man said) overpriced - plenty of oils out there better for the money. Use The harley Primary fluid and use a gear oil in the tranny. I still have my first Harley, a 1984 Ironhead 1000cc Sportster still looks great and runs great. I used M1 Car oil 15W/50 most of its life even in the trans/primary with no problems - ever. I now use a regular 20W/50 oil due to the word that some oil additives can hurt the clutch.
G-Man - I have been wanting to ask you why you were running 15W40 in your 883 with the factory recommending a 20W/50? Your results seem very good, however. I guess I was worrying about the 40wt part being too low. Did you notice a litte better mpg or power? I just dumped Rotella 15w/40 in my old Sportster because its what I had and never ride it anymore anyhow. Guess it would be okay for any short litte trips I take occasionally to keep the bike exercised.