Does a "rule of thumb" exist for when an engineer might specify a pressurized lube oil system (e.g., API-614 system) versus the simpler ring oil bath.

I suspect that at some energy density factor (horsepower x rotational speed), e.g., 10E06, pressurized lube oil is recommended.

Hi Bill,
Don't loose sight of the KISS (keep it simple ^%^%%#) principal. Generally the initial equipment manufacturer will identify he lubricant delivery system and then design the bearing accordingly. As you know as long as the rotational speed is sufficient to develop a lubricating wedge then the component can be oil ring lubricated. Sometimes you will need to use a significently thicker oil to develop that wedge. The question remains will the surfaced be protected against metal to metal contact during the development of that EHL film or is the surface area pressure sufficiently low enough to make the metal to metal contact acceptable (more surface area - more cost). If not then the necessity is there to provide for a pressure forced hydrostatic lift system. Some equipment uses a hydrostatic lift system for startup and then uses the ring lube system to maintain the wedge system. Some systems are dry sump and rely on the constant maintainment of an externally supplied hydrostatic lift. Others are mixed.
So no there is no 1 correct rule of thumb that I am aware of.

regards......