Hello folks, I just wanted to share my recent experience with the gearbox on my 2007 NC 6-speed Z-Sport. I have just got the car, and was a bit concerned about the really stiff and baulky gearchange from 1st to 2nd when cold. I decided to change the gearbox oil so that I knew exactly what was in there - even though the service history suggested this had been done fairly recently, so I did a bit of research and ordered up Red Line Synthetic MT90 GL4 from Opie oils
The gearchange quality has been transformed with the Red Line, and the feel is much improved. The shift is still a bit stiff when very cold, but overall, it is 100% better. I am a lot happier now, and just wanted to share my experience in case anyone else is looking at replacement oil types to try.
I have now ordered Red line GL5 type for the rear diff, and will get that in there ASAP.
Oh, and another bonus is all the Red Line bottles can double as nice funnels when empty - check out the shape 
Just be careful when ordering, as Red Line is sold in US quarts, so you need 3 bottles to do the job…
Red Line oils have a fantastic reputation, quite expensive but very much worth it!
Can anyone recommend the suitability of this oil in the 5 speed, or if not, what they would suggest based on experience?
I considered the Redline but with cost being on the high side I decided to stick with the Castrol Syntrans 75w 90. Have used it in all my MX-5’s Mk1 5 speed and Mk2 Mk3 6 speeds.
When you consider you need a tad over 2 litres the Redline stuff will work out expensive for a 6 speed, I can’t say what it’s like obviously for these gear boxes.
For the 5 speed Mk3 2 litres will do the job with a drop to spare.
Did the syntrans perform nicely in your six speed? What’s the cold performance like?
I’ve been bitten by using different oils in gearboxes in the past and have had the shift/change quality ruined until I’ve changed the oil again so I really want something that’s tried and tested…
Having said that, the redline is expensive but for how often the gearbox oil needs to be changed (I won’t need to do it again in my ownership basically), I don’t begrudge it as much.
I found the Syntrans made a difference, still notchy a little from cold going from 1st to 2nd something the 6 speed is known for. It takes a few hundred miles for it to work around, others have noticed this too from posts I’ve read.
Another consideration should be to change the diff oil, was once told no harm in doing this on a more regular basis especially if you do long motorway type journeys. I’ve done mine twice in around 25k miles. Castrol again Universal 75w 90 semi synthetic gear oil, less than 1 ltr should do the job easily.
Used Syntrans myself when I changed gear oil at the weekend, first impressions were good, especially from cold, notwithstanding the 1st to 2nd issue. Found the price and service from Opie to be good. Didn’t look at Redline though, but pleased with Syntrans.