I guess your all correct!
I changed to a detergant oil in a '68 5TA i used to have, and there was a marked increase in the amount of sludge build up every time i changed the oil. Which concerned me so much it was a factor in deciding to rebuild the engine, inside the cases it looked decidedly cleaner, now whether that was me trying to look for something i dont know, the Sludge trap wasnt that bunged up either 1/2 full, maybe it had been cleaned before?
I have no doubt about the cleaning effect of detergent oils, like Josh says. I can also remember when Duckams brought out there "Green" (green as in colour) 20/50 this was much malighened and slated at the time for being heavily detergent and at the time was reputed to not do "Old Engines" any good, this was supported by my father and father inlaw both old school motor engineers. Who by the way told me if a cylinder head isnt hardened enough after 40years putting in "Hardened valve seats" for unleaded petrol was a load of Bl***ks.
I think if you intend to run on a detergent oil if your bike isnt a later (Late 60's) bike, then probably change over after an engine rebuild, and certianly fit a after market oil filter.
For longevity its gotta be regular oil changes and an oil filter every time! Im not convinced carbon will destroy bearings maybe the particals of metal floating around in the oil might!
What want good when i was 19 i put Holts de coker into my B44 and it did a fantastic job of removing any compression that remained! an excellent cleaner! Perhaps i should
have decoked it properly, i wont tell you what my Father said after looking at the shed wall!!
As for Shoddy engineering!! wernt they all!! we all know bikes were built on assembly lines .... End of! and it doesnt take much to put them together right! i dont think they would have time to mate the surfaces as we have time to. I love my BSA's, but we all have short memories! most of my bikes were held together with checker tape (go faster) jubilee clips and fence wire, and what ever engine wasnt blown up! Matching numbers .... A miracle!
IKBA and Bar
Regards
Steve