This is what I found:
Lycoming Engines were originally equipped with a bypass valve which was controlled by a spring. Referred to as a spring and plunger type, it functioned as a result of the amount of pressure in the oil system. The spring-controlled bypass system was superseded by a system controlled by a Thermostatic Oil Cooler Bypass which reacts to oil temperature changes.
Lycoming referred to this as a viscosity valve. It basically bypassed cold, thick oil back to the engine before going through the oil cooler. When the oil warmed up, and was therefore less viscous, the valve closed due to lower pressure, and allowed the oil to circulate through the oil cooler. The Vernatherm replaced the viscosity valve and did the same basic thing, except the bypass was based on temperature, rather than viscosity, which was the real reason you want to bypass the cooler anyway. However, on my ECI O-360 engine on my first RV8, I had a 90* oil filter adapter, with a vernatherm opening machined in to it. The four attach bolt holes for the adapter were noticeably larger than required for the longer than normal 1/4” bolts. This made alignment of the adapter slightly variable. This variation made the exact alignment of the vernatherm very imprecise. My vernatherm was only contacting it’s seat in the adapter on about 66% of it’s circumference, which allowed some of the oil to continue to bypass the cooler, even when warmed up. I removed the vernatherm, and installed the ‘viscosity valve’ that I purchased from Lycoming (~$30.00). This one thing lowered my cruise oil temperature about 12*.
The viscosity valve assembly mounts in a machined opening in the accessory case near the oil cooler. It has a hex plug in it typically, and is mounted vertically. Very simple, poppet type plunger and spring. Remove the hex plug, insert the valve/spring assembly, re-install the hex plug.
Worked for me. I think I still have that vernatherm somewhere if you need one. Nothing wrong with it probably. Just a misaligned oil filter adapter. I’m no expert, so you should research this if you decide to do something similar.
After doing is little more research, it looks like the ‘viscosity valve’ bypasses the oil cooler, but also bypassed the oil filter. The vernatherm allows oil cooler bypassed oil to continue to flow through the oil filter. My 90* adapter issue bypassed the oil cooler, but still allowed the oil to be filtered all the time. My problem was higher than desired oil temperature because of incomplete vernatherm bypass valve seating.