NP435 and Ford 9" Gear Oil

[Jaymus]

Guys, I'm torn between many answers for this.

I wish I was stressing on what BRAND to use, not what WEIGHT.

I've seen things from synthetic 75W-90, conventional 80W-90, and also 85W-140 recommended for the 9", and 8.8".

WHICH WEIGHT?

Please, if you know, share the knowledge. My 9" is under a 4x4 F-150, 1986 model. After seeing the price of synthetics, I'm just going to go conventional. I decided to go with synthetic, lol. Pretty much, I'm looking at this for my 9"... either 75W-90 or 75W-140 Mobil1. I just don't know which weight. I could have swore in some other forum someone said 75W-140 caused scuffing in the 9"... but yet, others say it offers better protection. Who out there knows what is best?

I put Mobil1 75W-90 in the NP435 yesterday, and it is smooth as butter. I don't know what was in it, but it was thick, sludgy, and red. My guess was ATF, but I don't know if they make red gear lube. It was way thicker than the synthetic Mobil1 75W-90 I put back in. But, my transmission feels much better at all speeds. I didn't think there was a problem, before, I just wanted to change my fluids out.

 

[gomes512]

Not sure on the 9" as I have never had an application that used one. My friend has a 8.8 under his Jeep though that we transplanted in. He has been using Mobil 75w-140 in it and I to have run Mobil 75w-140 in my D35 rear before. Never have seen any scuffing or extra wear material whenever we drain and refill in either axle. His 8.8's bearings seemed perfect when he pulled the carrier. The 8.8 had roughly 70K on it before we put it in and the guy said he ran 80w-90 the whole time.

Lately I have been using Valvoline's durablend 85w-140 which is a syn blend I believe. I have been running that year round even in the Chicago winter. I have not noticed much wear metal in the fluid when draining which was surprising, then again I have not run a uoa. I recently pulled an axle shaft to check the outer bearings for wear (lubed by the gear oil) and have not noticed any wear marks or pitting.

I run the 140 weight since the manual says to run a 140 when under hard loads. Worked out well so far.

 

[Jaymus]

Thanks for the input. Here's the thing on the 8.8" rears. They started out recommending 80W-90 for them, then later on, changed it to 85W-140 or equivalent.

I think I'll go with Mobil1 75W-140 for the rear. Well, I'll say this... at the moment, I'm leaning towards 75W-140 for the rear, ALTHOUGH it was spec'd for 80W-90 I believe. That's the problem, I can't find what it really calls for. And, to think that later on with the 8.8" in light duty trucks, they changed the recommended weight from 80W-90 to 140. This is why I'm confused. This stuff is nearly $10 a bottle, tax and all, and so I want to pick a good weight and not worry about it. The 9" holds 3 quarts, I believe. I think the NP435 is a 3.5 quart capacity, and the 9" is a 3 quart capacity.

EDIT: BTW, I struggled over the Valvoline Durablend (synthetic mix) and Mobil1 full synthetic gear oils. The valvoline was around $2.50 to $3.00 cheaper, I think, but I ended up going with the Mobil1 75W-90 for the transmission (NP435) due to seeing the strong add pack in some UOA's here, and also excellent results within those UOA's. And since (only on day 2) it's done good in my tranny so far, really good, I figured I'd stick with that brand for my rear end.

EDIT2: Also, my 9", like most after the 60's, has no drain plug. I'm going to put a small hose (like vacuum hose) on an empty gear oil bottle, on the cone tip lid thing, and suction out as much as I can through the fill hole.

 

[gomes512]

Hmmm that is a head scratcher indeed. I would just stick with the 140 like you said. But perhaps a Ford dealer can maybe clear up the issue, if there is one by you?

If the Mobil is working out good for you then I say stick with it. I bet the 75w-140 will an excellent choice.

I always recommend biting the bullet and popping off the cover to get rid of the crud on the bottom that suction cannot reach. You are spending a good amount on the Mobil 75w-140. Might as well get the old fluid on the bottom that contains the most amount of wear material.

 

[Chris142]

Wouldn't SAE40 or 50 be a better choice for that trans?

 

[Jaymus]

Ford called for 80W90 and Dodge called for an engine oil such as SAE40 or 50, and I don't remember what Chevrolet called for their vehicles with NP435's.

It's been doing better so far, we'll see how it goes. It's just hard telling what was in it previously... so technically, water may have done better than what was in it before.

 

[deeter16317]

Dodge also called for 80w90 for the NP435...I had one in a 1974 PW. A 75w90 would be close enough.

As for the 9"...OLD SCHOOL. I'd think any 80w90 would be fine, or a 75w90 synthetic.