GL-5 in my NP435

[Jaymus]

Well, this is about the issue of GL-5 oils eating synchros in transmissions.

I've been running Mobil1 75W-90 in my NP435 4-speed with low in my 1986 F-150.

I was told that it could be detrimental to the transmission. When I changed mine, I also changed my wife's 2001 Eclipse 5-speed to the same M1 75W-90 (GL-5).

This was over 1.5 years ago. Immediately, her car shifted smoother, but within 2 weeks, her second gear and 5th gear were grinding. That's when I really read up on it, and I switched her to Redline MT90. (75W-90) It's been fine since.

People also advised me to change my truck, but, I'm asking if my bearings are even made of the material GL-5 is known to eat up. I didn't change mine out because it continued shifting smooth. It shifted better from the first minute and now, over 1 and a half years later, it's shifting even better. Does it have different bearings? This isn't your average transmission, anyways. Was a heavy duty trans put in many heavy duty vehicles.

So, what I'm asking is, should I be switching this thing to Redline MT90, or since it's caused no problems, am I safe?

 

[Gary Allan]

When was the NP435 first offered on the market? My SM420 was quite similar ..and then (in the 70's) you had no MT rating. There was 90wt and (trying to remember) posi-lube.

 

[Jaymus]

I believe it came in some vehicles back in the 70's? What is "MT rating"? In my manual, it calls for 80W-90. In a Dodge manual, it would call for an SAE 40.

I'm just wondering is there anything in my transmission that this GL-5 will hurt? So far, it's done nothing but improve. It's of course not a smooth shifting transmission like a little Honda, it's an HD transmission. But there is no grinding, and the shifts are smoother than ever. And it's been in there for over a year and a half with no problems, while it caused problems in my wife's Eclipse within 2 weeks.

Would I be wasting money by buying Redline MT-90 (or any other GL-4 synthetic) for this NP435?

 

[aquariuscsm]

Definitely put some MT90 in the Eclipse asap.

 

[Jaymus]

Yeah, I did. This was over a year and a half ago. 2 weeks the Eclipse started grinding. Put in MT-90 after than, and it was smoother than ever, although I think it did some permanent damage to the 5th gear synchro. You still have to be very easy with it (shift smooth at correct RPM) to not have it grind.

But, it's been in my truck's NP435 for the whole 1.5+ years... and I've had no problems. Any input on the truck, guys? Should I spend the money on 4 quarts of Redline MT90 if the GL-5 is actually, and has been, doing great in my transmission? I mean, normally it's a 'no-no' for transmissions.

 

[mechtech2]

Your wife's car was grinding not due to any wear/corrosion caused by the unsuitable M1 , but by it being too slippery for correct synchro operation.
This is why M1 does not recommend it in manual trannys, as well as it not being good for brass. And they are trying to sell it!
What does that tell you?
In your car, you got lucky, and probably have aggressive synchros.

Why take a fluid that even the Mfr. says is not right for manual trannys, and try and use it? There are many other better options.

 

[Jaymus]

I didn't know at the time. Jesus, like I said, I read up into it after it was said and done.

It's easy to criticize... I criticized myself, but I'm asking is it ok for this particular tranny. An NP435? This is not a car transmission. It's a HD truck transmission. First gear ratio of 6.68:1. Tough.. think T-18/T-19.

I'm wondering if it even has brass in it. Understand? It shifts like a dream.

The whole reason I got it is because I was looking for a fully synthetic gear oil that I could buy off the shelf, and M1 was the only one at Autozone, so I got it. I found out later about the GF-5 and brass parts, etc. This was over a year and a half ago.