Where is the optimal temp range for engine oil? With something like a 5w20 oil, I'm guessing there is a temp range where it is most efficient and is hot enough to remove contaminants. I'm asking this in regards to engine operating temperature. Different vehicles use different thermostats and engine cooling setups. In particular, my truck came from the factory with a 203deg thermostat. That has been replaced with a 190 deg thermostat. Along with the thermostat change, I have changed the cooling fan settings for turn on. Since there should be relation to operating temperature and engine oil temperature, I'm just curious to see where this all falls together. Before people starting arguing about lowering your thermostat temperature, I'll just say that with these engines, many, many, many people are running 180 deg thermostats and revised cooling fan settings without any know problems. I felt better with going in between the factory temp and the lower 180deg that are the most popular. Since there has been some talk about computer systems running richer with 180 deg thermostats, and an occasional code for slow warm up, I felt the 190 is a good compromise. Lowers the engine temp about 14 deg, without bringing it down a total of 24deg as some others are doing. As we speak, the engine comes up to 190deg, thermostat opens, temperature drops down to 182 or so, and repeats the cycle while driving as it should. Cooling fans are set at 199deg to turn on, and only make it to that point while sitting in traffic, or stop light, etc. While driving, the natural air flow keeps the engine running without the use of cooling fans. So, that all being said, my engine oil does not reach 200deg + temps, and I'm wondering if 180-190deg temp's are enough to burn off contaminants etc.