Let's start with some background.... The transmission (and engine) in my car (1990) is from a 1991 Miata and only has 91k miles. I saw the odometer of the car it came from so I know it is legit. It shifts incredibly well and doesn't make any odd noises. I installed the engine and trans late in 2010 for my rookie season in 2011.
The first decision I had to make was which oil use.
Deciding on a Transmission Oil
NEO Synthetic 75/90 gear/transmission oilAs part of maintenance and in preparation for the 2011 racing season, I drained what looked like the original transmission oil and filled it with NEO 75/90 synthetic gear/transmission oil. I had decided to try NEO for 2 reasons:
1. Based on feedback from various 3rd generation RX7 owners that I know
2. Because the transmission on my street/track 3rd generation RX7 had run Redline MTL its entire, long, hard life on race tracks across the US and the bearings were absolutely trashed when I had the trans rebuilt. MTL is pretty thin and slippery which makes for great shifting but some people have mentioned bearing wear on transmissions that have used MTL regularly. Granted, this is anecdotal but good enough for me for now...
I ran the NEO synthetic 75/90 on the street and track here in Northern California and it worked reasonably well. The transmission shifted OK using NEO. The one issue that I did have was on the 2-3 shift. It was a challenge to get that shift done smoothly during a race. I noticed this at Thunderhill in turn 5 and in Turn 11 and also at Laguna Seca in turn 2 and turn 11. My motor mounts (Mazda Competition) and diff bushings are in good shape so I couldn't blame driveline slop for this issue.
Time for a switch but which oil to use???
SwepcoI know a few local Spec Miata racers that use Swepco gear oil in their transmissions and differentials. I have been thinking of trying this oil but it isn't available locally so it is takes more effort to get my hands on it. I didn't do enough planning in advance to order some and wound up not using it but I suspect I will try it.
Redline gear/transmission oilI decided to switch back to Redline transmission oils. Redline actually makes two oils for manual transmissions - MTL and MT90. If you read your owners manual for recommended oil viscosity, you will notice that it says to use 75/90 in colder climates and 80/90 for warmer/hotter climates. At least, I think that is what I remember reading... I had tried Redline MT90 in my RX7 and liked it so I thought I'd try it in the Spec Miata.
Redline MTL is a 75/80 weight gear/transmission oil.
Redline MT90 is a 75/90 weight gear/transmission oil.
When I drained the NEO from the transmission, the oil only had about 4 events including SCCA Driver Licensing School and some street usage. It looked like it was in reasonably good shape and there wasn't a lot of metal "fuzz" or flakes on the magnet in the transmission drain plug. Too much metal shavings on the drain plug magnet means too much wear on the gears and bearings. I filled the trans with the Redline MT90 and also topped off the shifter turret - don't forget to do that!
ResultsSwitching to the Redline MT90 had the desired result improving the transmission's shifting overall. The transmission shifted smoother than the NEO and the fast 2-3 upshift was noticeably smoother in race usage at both Thunderhill and Laguna Seca. I wound up keeping it in there for the rest of the 2011 season and the test days that I did at year end.
Maintenance - Transmission Oil Change Intervals - Spec MiataWhat do racers do in the off season when waiting for the race season to start? Maintenance for one. I had about 8 race weekends on the transmission oil at this point so I knew I had to change it for the 2012 season. I briefly thought of trying Swepco but I forgot to order it and wound up going to Rotorsport since they are local to pick up some more Redline MT90 since they stock Redline, Motul, Royal Purple and a bunch of other race lubricants.
I drained the transmission oil and was surprised by the amount of "fuzz" from metal shavings on the drain plug magnet. I should have taken a pic but I usually make a mess when changing the transmission oil... There was too much metal shavings on the magnet than I was comfortable with. I am going to decrease the transmission oil change interval on my Spec Miata to 4 or 5 weekends. Hopefully there will be less metal on the drain plug next time around. If there isn't, then I will probably switch again. I wound up taking a sample of the oil and will be sending it to Blackstone Labs for analysis so that I have actual data on the performance of the Redline MT90 in Spec Miata race usage. Real data is better than anecdotal "evidence".
If anyone has input on transmission oil and oil change intervals, please comment.