shcm
September 12, 2008, 5:22 pm
[quote name='anchorman' post='797054' date='Sep 9 2008, 10:03 PM'][b]all I can say is trust me on this[/b], you will have difficulty getting gears when cold if you use a multigrade oil with a higher viscosity than 75![/quote]
Well, I don't know! Statements like that usually induce the opposite effect in me. However, as it is you, I will make an exception this time.

. You of all people should know that design engineers are a skeptical bunch. Well, usually the best ones at any rate

.
Anyway, I got curious. From my level of naivity, it would appear oil viscosity with temperature is a logarithmic/exponential (it's all the same to me) function, so with a few (two) data points you can make a crude first approximation of what the curves might look like:
[url="http://www.opieoils.co.uk/p-723-castrol-epx75w-80-gear-oil.aspx"]http://www.opieoils.co.uk/p-723-castrol-ep...0-gear-oil.aspx[/url]
[img]http://i208.photobucket.com/albums/bb315/shcmpb/saeminus20.jpg[/img]
Better scale for temperatures 0C and above:
[img]http://i208.photobucket.com/albums/bb315/shcmpb/saezero.jpg[/img]
So, yeah, it seems to suggest for example that at 0C the 75W-80 is "thicker" (915 cSt) than the straight 75W (410 cSt). No Surprise there!

Put it another way, 75W-80 in the box at 0C, is like having 75W in the box at about -7ish C, even though both oils meet the low temp 75W spec. Is this what you are getting at with low temperature gear selection?
What I don't have a feel for example is how 915 cSt compares with 410 cSt, or even how 7.4 kcSt would compare with 4.2 kcSt at -20C. The fact that the curve is exponential does make me wonder whether in reality the 915 v 410 doesn't feel much different.
Would 70W-75 be an option?
[url="http://www.opieoils.co.uk/p-1014-redline-superlight-synthetic-gear-oil-70w-75.aspx"]http://www.opieoils.co.uk/p-1014-redline-s...oil-70w-75.aspx[/url]
That's more expensive than your dealer supplied 75W though.
All of these have very similar high temperature values and cover similar "normal uk operating" ranges but the 70W-75 would be a bit thinner at low temp (again no surprise).
OK, this is a very basic, crude and possibly flawed analysis. I appreciate there are other factors. The stuff has to withstand high pressures etc. What else am I missing? Are there perhaps any materials in the gearbox that may not like some of the viscosity modifiers in a multigrade?
Anyway, if Kingo can do a deal, that's perfect!
Oh dear, I didn't want to turn this into an "oil spec" debate and I probably just have
Cheers