Which Oil Grade?

[towelie]

i used 10w-40 magnetec for a while, then changed to mobil one 0w-40.. after a proper rag in the car, if i stop the car then wait 5-10 mins an start the car again.. i get hla clatter for a few seconds on start up.. also since i changed i get blue smoke on the over run.. maybe its due to the thinner oil.. maybe not!

[skinny]

robgee1964 said:

Based on that reasoning the viscosity charts could be dispensed with, however there has to be a good reason why the manufacturers continue to publish this information. I wonder what happens to the viscosity of a 0W40 once you go above 100degC, compared with a 10W40?

as you've already said, the relationship between viscosity and temperature is non-linear, so at high temps, 10 degrees difference in temp is much less significant on viscosity than it is at lower temps.

however, with the viscosity on a log scale against temp, the line between teh kV40 and kV100 points is a straight one. given the 0W-40 will be less viscous than the 10W-40 at 40 deg, the gradient of the line is shallower and so at temps beyond 100 deg it will be more viscous than the 10W-40. you can also look at the HTHS values to get an idea of how the oil behaves at the higher temps although that's bearing viscosity rather than normal kinematic.

Edited by skinny on Tuesday 16th August 21:25

[jonzink]

guys my mazda 2004 1.8 has done 72k miles and will be used daily and for trackdays. waht shoukd i ise fully synthetic 10w 40?