Change trans oil or new transmission or neither?

I'm relatively new to my tacoma, coming from a manual transmission Mazda Miata (similar mileage both around 150k). The Miata could shift from 1st to 2nd at redline (6-7k RPM). The Tacoma I need to pause in neutral while the RPMs fall more or less all the way down to idle and then I can shift into 2nd fine. At a stop I can shift fine. If I try to do the 1 to 2 shift faster (at higher RPMs), the shifter might chatter/rattle in my hand and it doesn't feel happy about that, so I don't do that. All other shifts are fine. The transmission oil was just changed by the previous owner. I just bled the clutch and that didn't change things. I'm inclined to think it's probably a bad synchro. I've heard Redline MT90 can be good for this transmission (even though the manual says to use Hypoid API GL–5 75W–90 4WD 2.3qts and Redline MT-90 is GL-4). Since Redline isn't cheap, should I bother trying it? If the synchros are bad, should I keep driving it like it is? Will that hurt anything? Or should I get a rebuilt transmission? If I should get a new trans what else should I do at the same time?

 

Sounds like it may be the clutch. How many miles are on it?

If it's the original clutch then that's your problem, I'd bet money on it. 150k on a clutch is over it's expected lifespan. Some people manage to squeeze out more, but on average a clutch lasts 100k miles give or take.

 

It has 150k on it. I'm not sure if it's the original clutch. I haven't felt any slipping yet but I've only driven it 1500 miles. I did drive it in the mountains where I'd expect it'd slip if anywhere.

If it ends up being the synchros would you go with Marlin Crawler or used or rebuilt from another source? I can get a Marlin Crawler for $1400 or so, or a rebuilt one locally for $850, or used for maybe $400. Honestly I'm tempted to just replace the trans, clutch, freewheel, throwout bearing, pressure plate, all that stuff (what else???) and hope to get endless miles out of it.