Gearbox Oil Head Scratching

[G1en]

As some will know, I do a fair bit of track/race stuff in my 350z and one of the big problems this year was 4th gear. Now the gearbox is being re-built for the second time and the company doing it local to me (Its all they do so they know gearboxes in general inside out) have thrown a big spanner in the gearbox so to speak. They rang me today to say the 2nd and 4th synchros i had provided to be swapped, that the existing synchros looked A-ok and only 5th (which i did not do last time) looks like it needs doing, well definately needs doing they said. now i don't have a new 5th synchro to put in and thats not important really as I never use 5th on track, but they also said they think my main issue is the oil i am using.

 

Now I cant remember the last lot i put in, its probably on my build thread, I think it was Fuchs 75/90 titan stuff but that isn't really the point of this as I explained to him I change the gearbox oil twice a year anyhow and every time its something different but I try to get the "best" & "expensive" stuff i can find, This is the problem he explained, I am using oil with too low friction, its basically too slippy and not allowing the synchros to friction and bite! I am trying to understand why the more expensive an oil is, the more it offers "low" friction but the gearbox builder says this is bad and I should be using 10W40 engine oil or a cheap mineral oil (definately not synthetic) which kind of goes against what you would think, BUT does he have a point? does anyone know?

 

Also, the second spanner in the box is that online (i know i know) lots of forums stating DO NOT USE GL5 in a 350z box only use GL4 with some rational behind it stating the GL5 contains additives that degrade the brass Synchros, now I have definately been using lots of oils that state GL4 & GL5 like the one i am about to put in now below:

 

 

Like above, I thought as the box is being rebuilt again, I would opt for the best i could find, Motorsport use and states suitable for synchromesh gearboxes but is this going to cause more issues than it solves? 

Does anyone know if this is ok?, i am going to ring Millers tomorrow and ask their technical team.

But would also like to know if anyone definetively knows whether a GL4 / GL5 combi oil is no good or a fully synthetic is no good or that we should be using a cheap engine oil / mineral oil for track use (slamming through the gears at high RPM) as they offer a better coefficient of friction required for the synchros then chip in.

I may update this with future findings but no doubt the proof of the pudding will be when i do the first event of the season next year and how long it takes for me to swear at the gearbox!

 

[G1en]

Found an old topic on the site where opie oils recommended just this Redline product

So the Gearbox company may have a point, may need to get this?