Based purely on cost you would be best off going with used oil from your neighbors Winnebago.
I draw the exact opposite conclusion for someone who only changes yearly. Changing all 3 fluids takes a grand total of about 5 quarts in a Softail. Redline products (or pick any good synthetic), which I trust to continue to protect my bike, even if it gets very hot, much more than any dyno oil are maybe an extra $4 a quart. $20 a year is cheap insurance. Anything over 2000 miles a year and you are at less than a penny a mile.
If the bike gets hot in June and you cook the dyno oil, you would be running less effective oil for the entire rest of the season.
I don't see synthetics as a way to extend change intervals (they get dirty just as fast)....I see them as more likely to protect the bike for the entire interval than dyno oils.