Blending 90 and 140?

[JZiggy]

Hey guys,

I sometimes use a 50/50 blend of 75W-90 and 75W-140 in my rear diff as an attempt to hit a ~110 in the brand I like to use.

It occurred to me today that perhaps a "blended weight" is not the same. Here's what I mean: a given oil may have a distribution of oil molecule lengths with a single mean. For example, the 90wt oil might have molecules that are on average 1000 "somethings" in length while the 140wt would be 2000 in length. A 110wt would have a mean closer to 1500.

If I blend the 90wt and 140wt then I have a bimodal distribution of molecular lengths. Does hit actually behave the same as a distribution with a single mean?

Or does oil viscosity even work like this ... ?

 

[khittner]

Dunno, but 75w-110 is available, so why bother?

 

[StevieC]

I think a lot of folks over think gear oil in normal applications like a daily driver. I would use what your manual calls for or as close to it as possible if not available for some reason, change it at a reasonable time and not worry about it.

Diff's today are super reliable and will get by on almost anything not changed at all, so changing it with something as close to as what your manual calls for is far better and will ensure it should last virtually forever.

 

[dogememe]

If you really wanted 75w-110 I would just pick up some Amsoil in that flavor. I was gonna go that route in the Crown Vic before deciding to sell it.

 

[MolaKule]

A very simple way to determine the resulting viscosity is to send in your 50/50 VOA mix to Blackstone and check the resulting viscosity against:

https://www.bobistheoilguy.com/viscosity-charts/

 

[JZiggy]

Thanks for chiming in folks. Unfortunately overthinking it is what I do. I'm actually not really concerned about the application, I'm more asking if anyone knows how polydispersity affects oil viscosity. Interested to understand the theory is all.

 

[zeng]

Originally Posted by JZiggy
Hey guys,

I sometimes use a 50/50 blend of 75W-90 and 75W-140 in my rear diff as an attempt to hit a ~110 in the brand I like to use.

It occurred to me today that perhaps a "blended weight" is not the same. Here's what I mean: a given oil may have a distribution of oil molecule lengths with a single mean. For example, the 90wt oil might have molecules that are on average 1000 "somethings" in length while the 140wt would be 2000 in length. A 110wt would have a mean closer to 1500.

If I blend the 90wt and 140wt then I have a bimodal distribution of molecular lengths. Does hit actually behave the same as a distribution with a single mean?

Or does oil viscosity even work like this ... ?

The 140 wt (of 2000 in length) may play a more prominent role than a 90 wt (of 1000 in length) in the context of molecular weights, as its lighter portion gets pushed out of the way of gear contact zone whilst its heavier portion gets 'stucked' within the contact zone.Hence a thcker than 'average' MOFT in application.
Just as the higher polarity base oil, or the FM with lower COF of whichever sample, may play a more prominent role.
Most other properties such as fluid drags etc would likely behave as 'average' .

 

[benjy]

redline has a 75-110 as well + its the best IMO.