I had some idle time on my hands today so I thought I'd try something with my lawn mower. I was curious how two different viscosities would react in a air cooled engine with regard to oil temperature. I really didn't expect what I encountered. Are there any good explanations for these results? I have a 5 HP Briggs engine. I ran the engine for 10 minutes with Shell Rimula SAE 30 oil in the sump. It uses 20 ounces. The outside air temperature was 75ºF. The oil temperature after 10 minutes was 212ºF. I drained this oil and put in 20 ounces of Castrol 5W-20. I ran the engine for 10 minutes. the oil temperature after 10 minutes was 222ºF. I drained this oil out and put in 20 ounces of Shell Rimula SAE 30 once again. I started it and ran it for 10 minutes. The oil temperature this time was 207ºF. Why did the oil run hotter with 5W-20? Was the 5w-20 actually holding more heat from the metal and therefore causing a cooler running engine? Or was the SAE 30 actually keeping temperatures more in line?