If longevity without overhauling is the measuring stick here... the record on an Evolution engine is over 400,000 miles on HD 360 dino oil. There are at least two such engines which have been documented. And now Dave Willet (who put around 420K on his 1987 Evo) has nearly 300,000 miles on a TC engine (I don't know if he has done cam chain tensioner work)... He's still using dino HD 360 oil at 2500 mile OCI's. American Iron magazine has profiled these motorcycles, and Harley Davidson actually gave Dave Willet the new Twin Cam he is riding now, in even trade for the 1987 Tour Glide with the 420K+ miles on it. (the Tour Glide in now in the Harley Davidson museum in Milwaukee)... The services were all documented at HD dealerships around the country, and the oil was always the HD 360 stuff, some of it was probably the new Citgo forumula, most was probably the Sunoco produced stuff. Both oils work just fine.
So we can't use anecdotal evidence of 100,000+ mile engines to bolster the aptitude of synthetic oil over dino oil. Since we know that a properly serviced HD engine can run over 400,000 miles on dino oil without an overhaul, it should not be surprising that a syn could make that engine run at least 300,000 miles. But since the syns are turning in higher wear metal counts in UOA's... then I don't think any reasonable person should expect their engine to last
longer on a syn than on a good dino oil.
We can discuss the reasons for the wear metal differences, but we cannot dispute that they are there.
The noises we hear from our Harley Davidson engines are rarely a good indicator as to whether we're hearing a normally performing, happy engine or not. The molecular structure of some oils might (
might ) carry sound a bit better... or even the humidity level in the air can muffle sounds on the day we decide to see whether or not our new oil makes the engine quieter than the old one... the outside temperature has a direct effect on how quickly an engine warms up, and also how hot it runs, which in turn alters the volume and pitch of sounds coming from the engine. A full gas tank can dampen top cylinder noise, whereas an empty or nearly empty tank can echo these sounds...
Wear metal counts in UOA's should be the criterion. And there is where we see that a good dino oil seems pretty hard to equal for a syn--let alone beat.
Dan