the thinner the oil the faster it will break down at high temps. 5w20 will break down faster then 10W30. at high temps. gatta go and shut off computer....massive thunderstorm coming.........ahhhh.

 

heres an easy way to remember the numbers are the thickness the first is cold and 2nd is hot. the lower the number the thinner it is the higher the thicker it is. so when its cold outside you want a thinner oil thats one with lower numbers and when its hot you can use a thicker oil thats one with higher numbers. so will using a thick oil in winter kill your car? no but its not good for it. it will shorten the life of it if you use the wrong oil for to long. its thinker and the cold air doesnt help thin it so the engine has to work harder to get everything moving the harder it works the shorter its life with be. now why bother putting a thick oil in during the summer and not just use thin year around? because the thick oild will do its job better in the summer then the thin one. its job being lubrication or keeping metal from touching metal. the thinner oild will be to thin during the summer it would be like water and it wouldnt protect your parts fomr wear and tear of driving where the thicker oils will still be able to protect in hotter temps. now does that mean putitng a thin oil in during the summer will hurt your car? again not in the short run oil is oil in the short run it will do its job but not as well so over time there will be more wear and tear if you use the wrong one.