The difference in "slope" will dictate how the plots cross the mid temperature Y axis. Unfortunately, as said above, the SAE ratings are viscosity ratings at 100 C but to find the "slope" of the plots you need to know the ACTUAL viscosity at 0 C too. Hi George.Yep I realise that's what you were trying to achieve with the graph and initially I thought you had done this with the graph. With the chart you could then pick out the product that has the flattest range within the temperature band you intend to use it at. You would see:Ģ) Straight oils thin more quickly as they are heated.ģ) Multigrade oils thin less quickly as they are heated.Ĥ)Straight oils thicken moreas they cool.ĥ) Multigrade oils thicken lessas they cool.Ħ) The "High" viscosity figure of multigrade oils compares exactly to viscosity of straight oils at 100 Deg C.ħ) The wider the viscosity range of multigrade oils the flatter the decline (But it still thins as it gets hotter). I haven't got much spare time but it would be useful to draw up a plot of the range of oils of viscocity vs temperature for all types of oils. At normal average day temperatures of 15-25 C you will find these oils are probably as thick (or thin) as a 20W-30 grade oil.You might think 0W is too thin.but you have to understand is that the oil has simply not thickened as much as the 20 grade oil would have from 20C down to 0C. Ideally for oil damped forks you want the viscosity to remain constant rather than thin out, so to get a flatter range for normal temperature variations you could perhaps try the modern synthetic oils that can have a range of 0W-40W. The higher figure simply compares the multigrade oil to the equivalent straight grade oil's viscosity at 100 deg C so it'salways thinner or runnier than the cold figure rating. For example, a lubricant with an ISO grade of 32 has a viscosity within the range of 28.8-35.2, the midpoint of which is 32.Castrolite was a 20W-30 multigrade oil.so if you want the same viscosity range then choose one that matches.Īll oils including multigrade oils thin as they warm, many people believe for example a 20W-50 gradebecomes thicker as it warms.this is wrong. Each ISO viscosity grade number corresponds to the mid-point of a viscosity range expressed in centistokes(cSt) at 40☌. Many petroleum products are graded according to the ISO Viscosity Classification System, approved by the international standards organization (ISO). SAE 90 to 250 and 20 to 50 specified at 100☌.
Viscosities based on 96 VI single grade oils.Įquivalent viscosities for 100° & 210☏ are shown Viscosities can be related horizontally only
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