Botswana's 5000 hours (including 40 hours single engined!) operating experience on Bristol Frighteners seems pertinent, especially the use of AeroShell 100 U additive oil doubling engine TBO's to 2400 hourss.
Regarding recommended oils for Bristol sleeve valve engines, this ww2.aircraft.net forum link to seems relevant:
https://ww2aircraft.net/forum/thread...-valves.53855/
The AeroShell 100 U additive oil seems to be no longer available but the AeroShell W100 additive oil is available.
The AeroShell Book (page 3.1) explains the AeroShell W Oils:
To overcome the disadvantages of harmful combustion chamber deposits, a non-metallic, i.e. non-ash forming, polymeric additive was developed which was incorporated in blends of selected mineral oil base stocks, to give the range of AeroShell W Oils.
https://www.shell.com/business-custo...dition2021.pdf