Oil Question (what else?)

Silver Spring, MD
I'm looking for a simple explanation for the differences between major types of motor oil. My understanding is that a higher oil weight (e.g., 30 weight vs. 20 weight) is a thicker, more viscuous product. Is this true? If so, what situation (or engine type) would warrant a higher weight oil? With respect to the first number often associated with oils (e.g., 5W or 0W), my understanding is that this has something to do with its functional status at lower temperature. Is this approximately correct? If an engine can run with either 20 or 30 weight oil, what would the implications be of using one type (e.g., 5W20 vs. 5W30) or the other? Kent Kester
Columbus Ohio
You need to use a thicker oil, if your oil temp is higher. A 50 weight oil at 250 degrees, is approximatly the same viscosity as a 40 weight at 230 degrees, or a 30 at 210. It is common practice to use a thicker oil to reduce consumption in oil burning engines. A thinner oil will reduce friction and maximize heat transfer. It all depends on the application. I wouldn't use a 20 in anything that didn't call for it. Tooslick did some testing in that area, and it didn't seem to work very well. In vehicles that call for it, 5w20's seem to work very very well. [Smile]


Staff member
Oakville, Ontario
Originally posted by BIGJ552000: But if 5w-30 dinos thin to a 20w at the end of their 3,000 mile run...would'nt they be the same as the 5w20's [I dont know]
Not quite, because a 5w30 that thins into a 5w20 (or turns into a 10w20) will also now be getting the engine a lot more dirty inside, due to the breaking down of the viscosity index improvers. A 5w20 oil that starts out this way is better since it's not going to use a lot of viscosity index improvers to begin with.