hey everyone
as I am getting ready to change atf and gear oil in my 1985 4+3, i found that the Lucas oil i was going to use was not safe with yellow metals according to them - and since poking around a bit, that is mentioned as a concern for these 4+3's - I am still waiting to hear from some other companies but royal purple said that their Royal Purple Max Gear 75W-90 part #01300 would work great - does anyone have experience with this or have other recommendations for this - the fsm recommends 80w90 GL5 cannot remember or find my notes from when I changed it the last time

any help would be greatly appreciated

Sean

 

80w-90 GL-4 conventional gear oil from driven and Red line 75w-90ns GL-5. just came in if anyone has tried those instead.

 

[size=13px]80w-90 GL-4 conventional gear oil from driven and Red line 75w-90ns GL-5. just came in if anyone has tried those instead.[/size]

 

I've had very good results with Amsoil Synchromesh.

 

thanks - they finally responded last evening -they had recommended Severe Gear 80W90 - i will ask them about the syncromesh formula -

 

Been using Royal Purple for years works for me, years back the purple saved me big money on a 96 LT-1 Diff. was making noise drained the ORG. oil installed the purple stuff DA DA the noise stopped.

 

thanks everyone - do you know if I will need to do anything different if I ran a conventional or semi synthetic before - so changing over to fully synthetic - with either the 4speed or the +3?

 

I run Royal Purple engine oil in both the 85 and 76. (Both engines are far from stock.) Even run it in the turbocharged LS that's in my Suburban. I love it as engine oil but tried it in the Super T10 in the 76 a years ago. (Correct oil for that transmission) It just didn't shift real nice in that car unless it was well warmed up. It was even worse at high RPMs, no matter the temperature. Came out looking great when I drained it to back to a conventional gear oil and I don't have that problem any longer. It just seems that transmission and Royal Purple aren't a real good match, if that makes sense. I know that's borderline useless data as it is a sample size of one.
I never tried it in the 4+3 that was in the 85 or the ZF6-40 that is currently in it.

 

Which RP did you use in that T10?

 

I have been skeptical of the synthetics in the t10 - not sure why - the synthetics should be good for so many applications - but....i guess maybe if it was new or rebuilt and that's. what you started with - certainly in the newer transmissions born and bread for

 

It's been a few years now but if I remember correctly it was the 75-90.
It would grind if you tried to shift aggressively at higher rpms. It seemed like it was a little too "slick" to let the synchros do their job. (Could be that the synchros are a little worn with age. Hell, the car is five years older than I am and I make some pretty interesting sounds if I try to move too fast before I "warm up" now)
As I mentioned: I use RP in just about everything else and it perfoms great. It's just that one transmission...

 

Regular RP gear oil has friction modifiers for diffs with a LSD, those are not usually recommended for a manual trans and don't play nice with syncro's as
you correctly surmised.
RP says it's fine (and who's going to take them to court to argue the point( and that it has the MT1 spec but I doubt anyone is enforcing that claim anymore.
That's probably why it didn't shift super well, and I've heard that report before. They make a synchromesh type fluid for MT's that use that or ATF or
such. In the very least a gear oil for a MT should not be rated for use in a LSD equipped rear end for the best results. That's why I just bought the "cheap"
stuff last time for mine, and I wanted to see if it shifted any differently than the relatively expensive Driven GL4. Zero difference in early summer Midwest climate.
fwiw
If I ever get the thing to quit leaking I'll probably try the redline I've used in other old trans with good results. Not sure it's worth the bother in this old box though
tbh.

I've had it in a couple of diff's that have 250K on em now for most of their lives with no issue other than expected wear for the mileage. I think I put it in the 86
when I was initially going through it since I had it on hand.

 

I just serviced the 4+3 and differential in my 85 while I had it off the ground installing the rebuilt L98. I used Royal Purple 80 W 90 in it and seems to work fine. Granted I have only driven the car a short distance since the rebuild. Shifts nice and smooth and no noise from the trans or diff.