I can't tell you how many times I've read/been told 75W90 gear lube is the same viscosity as 5W-20 motor oil. I'd like to put that to bed and move on. (pending correction by those that know more than I). However, Mr. Widman, who's data is used below, is mighty good himself. Let's look:
As you can see the bottom of 75W gear oil viscosity starts at 10W, and it could be as thick as 20W and beyond (lets call that 25W for now). At no time is it ever in the 0W to 5W range. None. Obviously And the potential range is quite large as well, so it's clearly not safe to say "any 75W90" is....
OK, how big a deal is this? Here are some reportedly average numbers:
On just a mildly cool day you're seeing about 230 cSt for 75W90.
On that same day we're seeing 5W30 (I don't have 5W20, but it's likely lower) at roughly 170 cSt.
So not the same right? Is that significant? I'd need Mola to tell us, for I certainly don't know. I can see it sure seems like 10W40 is the closest cold match to 75W90. (again the viscosity range of 75W90 is so wide the data could also be skewed as a result).
Now this most certainly doesn't address sheer strength or additive packages of a good gear oil, but that just not a viscosity discussion. Not do gear oils usually see the kind of temps an engine carry's so heat isn't likely to be as significant an issue as correct viscosity.
At the very least I hope we can agree 75W90 is not generally the same viscosity as 5W20. I do want to leave open the probability that there are some gear oils, perhaps synthetic oils, that could meet or approach that of 5W/20. Redlines 75W80 seems to exceed the 5W/30 threshold, as does their MT fluid. I'm sure there are others too. Note however they are GL-4 oils, not GL-5. Nor does that suggest they are getting to 5w/20 weights.
As a tangent, I have found a GL-5 rated oil that gets close to 5W/30: Redline 75W85 is slightly above 5W, but below 10W in cold, and beats both in both sheer and 100C temp. Close enough I guess we could say that at least one 75W85 Gl-5 rated oil is "close enough" to 5W-30. 5W-20? I don't know. If I can get Mr. Widmans graphing tools working I'll post the graph.
Respects to all,
-d
As you can see the bottom of 75W gear oil viscosity starts at 10W, and it could be as thick as 20W and beyond (lets call that 25W for now). At no time is it ever in the 0W to 5W range. None. Obviously And the potential range is quite large as well, so it's clearly not safe to say "any 75W90" is....
OK, how big a deal is this? Here are some reportedly average numbers:
On just a mildly cool day you're seeing about 230 cSt for 75W90.
On that same day we're seeing 5W30 (I don't have 5W20, but it's likely lower) at roughly 170 cSt.
So not the same right? Is that significant? I'd need Mola to tell us, for I certainly don't know. I can see it sure seems like 10W40 is the closest cold match to 75W90. (again the viscosity range of 75W90 is so wide the data could also be skewed as a result).
Now this most certainly doesn't address sheer strength or additive packages of a good gear oil, but that just not a viscosity discussion. Not do gear oils usually see the kind of temps an engine carry's so heat isn't likely to be as significant an issue as correct viscosity.
At the very least I hope we can agree 75W90 is not generally the same viscosity as 5W20. I do want to leave open the probability that there are some gear oils, perhaps synthetic oils, that could meet or approach that of 5W/20. Redlines 75W80 seems to exceed the 5W/30 threshold, as does their MT fluid. I'm sure there are others too. Note however they are GL-4 oils, not GL-5. Nor does that suggest they are getting to 5w/20 weights.
As a tangent, I have found a GL-5 rated oil that gets close to 5W/30: Redline 75W85 is slightly above 5W, but below 10W in cold, and beats both in both sheer and 100C temp. Close enough I guess we could say that at least one 75W85 Gl-5 rated oil is "close enough" to 5W-30. 5W-20? I don't know. If I can get Mr. Widmans graphing tools working I'll post the graph.
Respects to all,
-d