Brain fade re oil viscosity
Discussion
samoht said:
Super Sonic said:
It resists the thinning effect of being heated, so at a low temperature it has the equivalent viscosity of a thin oil, and at a higher temperature, it has the equivalent viscosity of a thicker monograde oil that has thinned with temperature.
This.It makes more sense if you think in terms of what people used to do before multigrade oils. In those day you might fill with a 5 weight oil in October for the winter, and change that to a 30 weight oil in March for the summer season. Ie actually put in a thinner oil in winter to lubricate better in cold weather.
The multigrade oil offers a single oil which can work year-round, so is advertised as "5w/30" meaning it is like a 5 weight oil in winter but a 30 weight oil in summer, saving you changing the oil twice a year.
Everything I read says the 5W/30 mubers are the viscosity so how is the viscosity lower in winter than in summer? CaddyShack above said the numbers mean flow not viscosity. Now flow would make sense to me as I'd expect the flow to be lower in winter (5) than in summer (30).
So is CaddyShack right that actually the numbers mean flow? So why call it viscosity? Appreciate I am being dense here as it is obvious to everyone else!
Thanks all. At times I have become more baffled by this evolving thread but some of you put your finger on it. But I had to look at the graphs available here to understand it myself - https://www.widman.biz/English/Tables/gr-motores.h...
Basically a 5w/30 oil is as thin as a "5" at cold temps and as thick as a "30" at hotter temperatures. BUT overall the multigrade 5W/30 is still thinner at the hotter temp than it is at the lower temp (this is what I wasn't understanding).
A multigrade oil is a clever thing then if can change its inherent viscosity as temperature increases (resulting from the coiled spring analogy someone gave previously).
I always thought oil was oil and didn't care if i was topping up with the wrong grade or not but now I can see that the wrong grade could make the engine work too hard when cold and the oil be too thin when hot thus potentially causing premature engine wear. I never thought the grade of oil was that important. Perhaps it's a lot more important than I had thought.
Basically a 5w/30 oil is as thin as a "5" at cold temps and as thick as a "30" at hotter temperatures. BUT overall the multigrade 5W/30 is still thinner at the hotter temp than it is at the lower temp (this is what I wasn't understanding).
A multigrade oil is a clever thing then if can change its inherent viscosity as temperature increases (resulting from the coiled spring analogy someone gave previously).
I always thought oil was oil and didn't care if i was topping up with the wrong grade or not but now I can see that the wrong grade could make the engine work too hard when cold and the oil be too thin when hot thus potentially causing premature engine wear. I never thought the grade of oil was that important. Perhaps it's a lot more important than I had thought.
TwinKam said:
It was explained to me once to imagine synthetic multi-grade oil molecules as coiled clock-springs and, as they get hotter, they uncoil, hence increasing their 'size'.
I had no idea what you meant when you said this but now I get it. Basically the "uncoiling springs" stop the oil getting as thin as it otherwise would at higher temperatures. TYGassing Station | Home Mechanics | Top of Page | What's New | My Stuff