With our 2003 Prius, I changed the oil, transaxle oil, knocked the air filter clean and cleaned the throttle body. After 3-4 years, the 1.5L engine throttle plate and body can develop a layer of carbon-oil junk that in cold, wet weather can keep the car from starting. Using a good carborator cleaner and brush solves the problem for another year:

The carbon-oil build-up is that semi-circle just behind and to the right of the throttle plate shaft. I suspect it comes from blow-by from the PVC tube.

The transaxle oil had 27,000 miles of service and looked pretty good. It was not opaque and smelled like oil. I'll be sending it for analysis.

Bob Wilson