Rear differential fluid advice needed

[Gros Ventre]

In the summertime a 140 weight lube flows like a 140 weight... In the winter, until its warmed up it'll flow like a 75 weight lube. When warming up the multi-viscosity lubes will flow somewhere between the two numbers. The key is simple when fully warmed up its a 140 weight which flows more slowly than 80 weight... Like I said above bearings, gears, etc are designed for a certain viscosity. If you go too far afield you may not break something, but may shorten the life.

Sponsored

 

[Mosey]

 

[FaaWrenchBndr]

In the summertime a 140 weight lube flows like a 140 weight... In the winter, until its warmed up it'll flow like a 75 weight lube. When warming up the multi-viscosity lubes will flow somewhere between the two numbers. The key is simple when fully warmed up its a 140 weight which flows more slowly than 80 weight... Like I said above bearings, gears, etc are designed for a certain viscosity. If you go too far afield you may not break something, but may shorten the life.

I completely agree with this…….a solid statement

 

[adaycj]

It's a personal decision as to which weight you use. Just to help clarify though 75w-140 is not a 140 weight oil. These multi viscosity oils are formulated so that at around -30 celsius I think, they flow like the lower weight would at that temperature, and at 100 celsius they flow the same as the higher number oil would flow at that temperature.

So, a 75w-140 oil vs a 75w-80: both will flow the same as a 75 weight would at the lower test temperature (-30 Celsius?), and one will flow the same as an 80 weight would if it were 100 celsius, and the other will flow like a 140 will at 100 celsius. At 100 celsius, both are going to flow pretty well and may be "thinner" than a 75 weight at room temperature. You'd have to investigate the details to determine this.

I ran 75w-140 in my last f-150 with zero issues. Generally the thicker formulation is recommended for heavy loads/high stress on the gears since the film strength is stronger and this will reduce wear.

Motor oils are the same. The lower number represents flow equivalent at the lower test temperature vs a single weight at the same -30 celsius temperature , and the higher number represents flow equivalent to that single weight if both were at the higher 100 celsius temperature.

It isn't magic. It's just a multi grade SAE gear oil. 75W140 is thicker than 75w80 (or 90) at warmed up operating temperatures which is what we are talking about. Plus my 203f for the hot point IS (about) 100c which makes the viscosity even more relevant here.

 

[adaycj]

Gears and bearings are designed for a viscosity range. There are water lubricated bearings that last 30+ years. To put in a 140 weight lube in a component designed for around 80 weight is not a good idea.

They are but the exact gears and bearings in these exact housings (9.75 and super 8.8) have been used by Ford with both 75w80 and 75w140 specified as the factory fill. My observation from having my hands on quite a number of failures over the decades is that in this case the fluid hasn't changed the failure rate. I'm currently of the mind that cooler with 0.02 mpg improvement is better, but other than temps I have no real data.

 

[ksdon]

If a person wants to run 75w-85 Redline seems to be a good product in the same weight at Ford's. Their literature says it meets Ford's WSS-M2C942-A. It is about $10-12 per quart less expensive than Ford. But, about $5-7 more per quart than Valvoline 75-90.

Sponsored