[SpencerC_6MT]

TL;DR: changed gear oil from MTL to MT-85, have to double-clutch when cold but synchros are happier at operating temp

Reviving this thread from the dead since its been a while. So yes, it turns out I did strip my fill plug 3 years ago, and I had a shop finish the job for me. I've been using Red Line MTL for the past 26k miles, and recently changed the fluid again to Red Line MT-85. This is against the advice of Red Line themselves, and I'll explain my thinking behind this decision in the following paragraphs.

My experience with MTL was pretty good, in short. Immediately I could tell the transmission was happier than with the OEM fluid once up to temp. However, as time went on, I began noticing issues with its performance in the cold. 1-2 shifts took a little convincing sometimes, and sometimes 3rd or 4th would lockout on the first try. Instead of changing the fluid, I actually decided to change my driving style, and I've been double-clutching every time the car's cold for the past year or so. Breaking the habit of driving normally is a little difficult at first, but after 10k+ miles of doing things this way, it's not so bad. I plan on keeping this car as long as the chassis is safe for public roads, so preserving any consumables (like the non-serviceable synchros) for as long as I can is ideal, and I find double-clutching for 5 minutes a small sacrifice.

So if I'm happy with MTL, why did I choose to go up in viscosity, especially when Red Line recommends their less viscous DCT-F? Well, as I gradually upgrade my car and improve my driving, heat and stress on the driveline is naturally going to increase in turn. Around half a year ago when I took it to an 1/8 mi drag strip, the car only managed 1 max-effort run before it started to grind 2nd. This is the only time it has grinded 2nd in a long time, and I suspect it's due to the gear oil getting too warm and losing its lubricative properties. Of course I wasn't being kind to the synchros at all, but it made me question how much extra wear I could be putting on the transmission while driving aggressively on backroads, which doesn't produce a grind. Considering I already double-clutch from cold, I figured I had more to gain by using more viscous fluid than I had to loose.

My experience with MT-85 hasn't been particularly notable since I changed it 4 days ago, but I'd say it's been positive. In terms of gear lockout, it's roughly the same as before. Sometimes 1st doesn't like to go all the way in from a standstill, and 3rd/4th will talk back a bit before it's all the way to operating temp. But once it's at operating temp, I'm happy with the results of the change. Resistance from the shifter is unchanged in regular driving, and resistance is similar (but better) when you shift it hard. The best example would be downshifting into 1st, since that's the hardest on the gearbox. Before, a downshift into 1st around 15-20 MPH would take half a second of pressure on the shift knob before going into gear, due to the synchros... yknow, synchronizing. With MT-85, you still need to give it time, but you don't need to press as hard, and it's a much smoother feeling. In other words, the synchros produce a bit more friction with the same amount of force applied, so the gear speeds align quicker. This is an indicator that the MTL -> MT-85 switch had its intended effect: allowing the synchros to do their job more quickly with less force. Although the gear oil may be too thick at cold temperatures, I circumvent that issue with my driving style, and I'm reassured by the advantages at operating temperature.

This change definitely isn't for everyone. In fact, I'd say most drivers are better off stepping down in viscosity to Red Line's recommendation of DCT-F. But if you like to indulge in bullying your synchros, especially if you're tuned, I'd recommend the step up to MTL or MT-85.