Concerns with full syn rear diff fluid; pinion seal leak?

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Charlotte, NC
I am going to change the rear diff oil in my 2006 Toyota Tacoma, 2WD. It is an open diff, I do not really do any towing or hauling etc. So, I was just going to put regular Valvoline dino 80W90 in. But Napa has the Valvoline full syn 75W90 on a good sale right now, making it close to the same price.
A couple years ago, I changed out the rear diff oil in my 2004 Ford Explorer. As per a TSB, I installed Motorcraft full syn 75W140. Some time passed and then my pinion seal started leaking. Bad enough that it was slinging the oil around.
I never thought about it until I started thinking about changing the oil in my Tacoma.
Do you suppose the full syn fluid deteriorated the seal in my Explorer?
Has anyone run into this before?
 
I am going to change the rear diff oil in my 2006 Toyota Tacoma, 2WD. It is an open diff, I do not really do any towing or hauling etc. So, I was just going to put regular Valvoline dino 80W90 in. But Napa has the Valvoline full syn 75W90 on a good sale right now, making it close to the same price.
A couple years ago, I changed out the rear diff oil in my 2004 Ford Explorer. As per a TSB, I installed Motorcraft full syn 75W140. Some time passed and then my pinion seal started leaking. Bad enough that it was slinging the oil around.
I never thought about it until I started thinking about changing the oil in my Tacoma.
Do you suppose the full syn fluid deteriorated the seal in my Explorer?
Has anyone run into this before?
Correlation is not causation.

Some time passed, during which your seal got older, and started leaking.

On my old 4 Runner, I changed from conventional to synthetic at about 100,000 miles.

At 300,000 miles, still no leaks.
 
The oil is not going to cause a pinion leak unless the seal was on its last legs to begin with.
A long time ago, I changed out the factory fill on a E60 528i with Amsoil 75W90. Factory fill had 180K+. Within a few weeks, the seals started seeping.
 
So far we have one guy whom said that it did not happen to him and one guy whom said that it did happen to him.
 
Do you suppose the full syn fluid deteriorated the seal in my Explorer?
Synthetic gear lubes will not damage a seal. What damages a seal is heat, wear, abrasives, and lack of preventative maintenance.

A seal that has been worn or damaged may have had a false seal of sludge and crud form around the lip and then when a synthetic lubricant was introduced with its increased cleaning properties, the lube removed the crud and exposed the worn seal.
Has anyone run into this before?
I have not.
 
So far we have one guy whom said that it did not happen to him and one guy whom said that it did happen to him.
You could try conventional and see if it leaks after the change.

In all honestly, a pinion seal replacement isn’t the end of the world either. If it happens, fix it.
 
A seal that has been worn or damaged may have had a false seal of sludge and crud form around the lip and then when a synthetic lubricant was introduced with its increased cleaning properties, the lube removed the crud and exposed the worn seal.
I was actually thinking this with my Explorer. I remember taking a hydraulics and pneumatics class at technical college back in the early 90's and the instructor explaining this exact scenario.
It is possible this is what happened to @The Critic as well and his BMW.
 
I was actually thinking this with my Explorer. I remember taking a hydraulics and pneumatics class at technical college back in the early 90's and the instructor explaining this exact scenario.
It is possible this is what happened to @The Critic as well and his BMW.
FF was probably a synthetic product anyway, so not sure if that is possible?
 
Likely began leaking due to the scenario described earlier. New fluid cleaned crusty ring next to seal lip and further exposed old, hard, worn seal. Sometimes it is best to stick with conventional lubes in older vehicles….
 
If a pinion seal starts leaking don't forget to check your vent tube. They plug up all the time and most people don't even know they exist. Especially if you're in mud dobber country, run a lot of gravel roads etc etc.
 
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