oil

Hi, can anyone tell me if any problems could occur mixing 0w 40 with 5w 40 oil only my recent service used the later whereas i still have 3 litres of the first, thanks

none at all, very little difference between the two, the 0-40's slightly more fluid at 0deg. C, with a 50/50 mix you'd end up with say 3-40.

I think the key thing is that they are both the same types of oil - ie fully synthetic or part synthetic. I believe the 2 may not mix well & then you may lose significant oil performance. By the way, the first part of the rating is the high temperature performance NOT the cold performance - ie 0-40 has very low viscosity when hot & 5-40 has slightly higher viscosity when hot. Both oils have the same "40" viscosity when the engine is cold. Very low viscosity is great for performance & getting the oil to all the critical bearings very quickly after start-up, but is potentially not good for bearing life or leaks. High viscosity gives lots of bearing protection when it finally get there, but creates "drag" which reduces performance & mpg. Paul 2004 996 C2 40 Anniversary

Sorry Paul, but i think you have got that the wrong way round. Both 0W-40 ant 5W-40 have nominally the same viscosity when hot. 5W is thicker than 0W when cold.

Many thanks guys, i always use fully synthetic and will therefore use up what i have first. Always used mobil 1 but for some reason the service used valvoline!!! I want to ensure i continue with mobil 1 so will use what i have, thanks for the technical info, David

Hi Richard, So, is the "5W" part of the rating measured on a different scale from the "40" part? - because obviously the oil must be less viscous at high temperature. This has always confused me about oil ratings. But like you I have always understood the "W" part to be the Winter viscosity. Or does "-40" means "minus 40"?! Mark

The first number is the viscosity at 40 degrees C, and the second is at 100 degrees C. The "0W", "5W" and "40" are in fact ranges. When comparing viscosities of oils, it is best to compare the kinematic viscosities in CentiStokes, which is a true value for the oil's thickness. Hopefully this chart will make it a bit clearer:

Here is a table I made some while ago from oil datasheets. You can see that because "0W" and "5W" are ranges that overlap, you can get a 5W oil that is actually thinner than a 0W at 40C.

Richard, You're right, I hadn't understood the oil ratings correctly. However, after your last post I think I understand ! Your last chart shows what I expected, that all of the oils have higher viscosity cold than hot - which I guess most people would intuitively think is right - hot oil is "runnier" (to use the technical term !). I'd therefore (incorrectly) assumed that the lower rating must indicate the hot viscosity. From what you've explained & shown in chart in your first post, the rating for hot & cold viscosity is totally different - even if the viscosity is still measured with the same Cst units. Its good to learn something new every day ! Paul 2004 996 C2 40th Anniversary

Very interesting. In that case you really can't compare between brands on viscosity - only within the Castrol brand (the only brand with more than 1 oil in the table) is there any relationship between the viscosity and the ratings. I'd be interested to see the viscosity figures for Mobil 1 5W-40 if anyone has them to compare with the 0W-40, but only out of "interest" - I'm not sure that I'll do anything different![8|]

I got the info in the table from datasheets on the Opie Oils site, and from manufacturers sites. You are right, it is very difficult to compare brands. Mobil 1 0W-40 is thicker than a Castrol 5W-40 in both hot and cold ranges.

omg, I didn't realize my post would get so technical. You guys certainly know your stuff. It will take a little longer to sink in with me but thanks all the same.