2. My oil pressure is somewhat higher and the engine seems to run a bit quieter when I use 40 weight oil, or even 20-50 in a multi-viscosity. Is there a downside to using heavier than recommended oil?

We prefer straight 30 weight detergent oil in the Atomic 4, simply because the engines are usually used at a constant power setting, and mostly during the warm part of the year (which doesn't really require a multi-viscosity oil). Later Universal owner's manuals did list multi-viscosity oils as a suitable substitute to straight viscosity oils, but we can't find where they ever recommended over a 10-30 weight.

The downside to using heavier than recommended oil is that (while the pressure might be reassuringly high on the gauge in the cockpit), the head loss in oil pressure between the port where it enters each bearing in the center, to the outside of the bearing, is much greater as oil thickness increases. This means that the outer edges of each bearing (crankshaft and cam) is getting somewhat less than designed oil pressure. - Updated: May 26, 2004