Royal Purple 75 90 LS in a 365 boxdifferential OK

Having replaced the gearbox/differential fluid on an '84 308 QV with Royal Purple maxgear 75-90 LS, and been amazed with the results, I am now contemplating the same for my '72 365 GTC4.
Understanding that seal technology in 1972 probably wasn't what it was in 1984, and the fact that I have NO LEAKS at this time in the box or rear, is this a good thing to do? 2nd gear's a little slow, as is 3rd, which I've heard is common in this 'box.

Hi Mike,

no takers so I'll kick in. a 365 GTC4 also ahy


The seals are a non issue, as they are actually of the same type still used on many cars today.

the slow shifts could be oil related.

Personally I can't think of any reasons not to change to a modern type oil. I changed a Daytona over to RP about a year & a half ago and the owner was pleased with the results at the time after a weekend of trying it out, and has never complained since.

Regards, JRV

Thanks for the reply, John.
As I have jumped out of the pan and into the fire, so to speak, with this model.... am trying to be extra careful.

Will post some pix and a little history next week.

I have a C4 and I just bought a case of RP 75/90 – I have heard nothing but great stuff about RP so hear goes.

I have swepco (sp?) in there now – both the differential and the gearbox. The differential is due – the transmission is just an upgrade.

My trans has new syncros and sliders – I put them in about 2 years ago. When hot it definitely works better but still shifting up to 2nd and 3rd – I have two options. –
Slow, gradual push and let the syncros work (1.5 seconds) or hard and fast (yes, When I slam it into gear, it works great!) It is the medium pressure shift (.8 seconds) that doesn’t allow for full rpm synchronization.

Down shifts are flawless.

Base on what I believe the design goal was – (use it hard and replace the parts often) I would say it works as designed.

A hard shift will cause the half moon springs under the syncro rings to engage and push the syncros out. This will add to syncro force and cause “good shifting”
In a causal shift, the half moon springs do not engage and the syncro force is only due to the syncro spring so you must shift slow enough to allow for synchronization with just this spring force.

Lower gears are worse due to mechanical ratios.


(As a side note – you never want to make a hard shift on a cold transmission with Porsche style syncros (which is what many of us got) – the internal friction from the cold gear lube will restrict movement of the internal half moon springs that expand the syncro rings and reduce the ability of the syncors to work – can result in big time grind or if the mechanism is force to work, the cold thick lube puts added stress on the syncro mechanism and will cause lots of unnecessary wear.)


I hope the RP will help the casual shift situation a little for a person who chooses to generally be gentle and pursue longevity.

Aaron

From what I have read from other peoples posts, the synchros don't like the limited slip additives, so if you have a separate diff and gearbox, then it would probably be best to use a gear oil with no limited slip additive at all. Does RP make anything like that?

Dave,

I have only heard good things about “purple”, and no word on issues of additives and Synchros. The Max Gear is supposed to, according to the purple people, be wonderful for limited slip difs and everything else.

All I can say is – we shall see.

What I like about web sites like this is that we all get to learn from the experiences of others. Rest assured that if I don’t like the “purple” for some reason, I will post a conclusion with specifics.

Have fun,

Aaron

JRV just changed to RP on my 308GTB.

The car feels good!!!! Sounds "happy" to me!

plus, they were just granted a trademark by the US on the color "Purple" per Autoweek

>>they were just granted a trademark by the US on the color "Purple" per Autoweek<<

Wonder what Welch's Grape Juice will do now?

Aaron,
Agreed, I intend to try Royal Purple this summer, when I am due for another change, but in your case, there is no need to "compromise" on a fluid that works for both applications, since the gear box and diff are separate, compared to the 3X8 series. But regardless, do report back.

Thanks

I guess I could call this the Purple Report.

The car is a ’72 365gtc/4 with low mileage synchro rings and a new slider for first and second.

I took out all the Swepco mineral based gear lube and put 5 quarts of Purple Max Gear – in the gearbox and the differential. 3quarts and 2quarts respectively – that is all they hold.

At this point I believe there is a slight improvement in synchronization with a warm gearbox and slow shifts - It is not significant. For fast “Ferrari style” shifts and a warm gearbox, the shifting feels less cushioned, and I seem to hit the stop harder, but again, we are splitting hairs and it may be that it was just before I had my coffee.

The most significant difference is when the gearbox is cold. When cold, the purple works better and like the Red Line I had in there a while ago, I think this is because of the initial low viscosity.

That’s what I’ve got for the first 40 miles. – This is my third experience with high quality lubricant – they seem to all work - imagine that.

At this point, I’m more interested in experience with longevity. Does anyone know of a long-term downside to using synthetic or mineral based oil?

Comments?

Aaron