Grinding gears from 1st to 2nd, 3rd to 5 ok

If it grinds when trying to shift, it is the synchronizers. Rebuild. If it pops out of gear. there is a plastic part at the top of the transmission that wears out. It fits around the ball of the shifter. It is two pieces. You'll have to remove the floor plate and pull it up to access the bolts to remove the shifter and replace those plastic pieces.

4th gear is not a gear as such but a coulping of the input and out put shafts to make a complete shaft
If there is a problem moving this coupling it indicates a syncro ring problem or spigot bearing failure in the end of the input shaft
either way it amounts to a gear box removal and pull down for repairs.

Check the fluid level in you slave cylinder. If the fluid is low or there is air in the system it can cause hard shifting. If everything is ok it is problably the syncros.

don't shift into 1st until stopped and 10mph or less is your down shift range to 2nd gear from 3rd

The 3rd Gear Synchro is worn or damaged, which is why it grinds when you try to upshift or downshift into 3rd gear. When it goes out completely you will have a very hard time shifting into 3rd gear, since the Synchro lines up the gears to make shifts smooth and easy.

Your pressure plate must be weak and about to let go.

What is happening to you is worn out sincronizers in you transmission. They are used to mesh the gears between shifts. They are made of brass and wear out easily on the 2nd gen cars. It grinds normally going into 2nd gear and into 3rd if its really worn. If you want a stronger transmission buy the 93 and up 3rd gen tranny. It is an upgraded transmission with dual sincros. Yours does need rebuilt. Or you can find one on the MR2 board for a good price used. I prchased a 93 trans for $1000. An older one will be about 200. Also it depends if its turbo, lsd or not. Good luck!

Check the fluid! Had similar problem, $0.87 in fluid fixed it.

With ALL due respect, I speak from a 30 year Career of AUTO & Heavy Truck Experence of Fleet Management and Personal Customer Auto Care, and by NO means do I mean any Disrespect to you here.
As Long as is Does Not grind while UP SHIFTING you R fine , DOWNSHIFTING into 1st Gear BEFORE 20 mph is a Disaster Waiting to Happen EXAMPLE> Just BECAUSE a car CAN run at 120mph does not Mean One SHOULD do it. If it downshifted to 1st gear before 20 mph with OTHER Clutch that was due to WEAR, there MAY be a bit of AIR in the Release bearing or Slave cylinder, That needs to be BLED OUT, DOWNSHIFTING IS NOT A recommended Practice Unless its an EMERGENCY STOP condition, as It Causes Premature Wear on Clutch Disc. I Know this MAY seem UNTRUE as you may have done it for years But IN FACT A Clutch is MADE to Simply Get From LOWER GEAR TO HIGHER GEAR, but if you Downshift ALL of the time you WILL Subject the Clutch to TWICE the WEAR you Should THUS WEARING it OUT TWICE AS FAST $ thats $$$$. A good number of shops wont Advise you this Far as it Would REDUCE THERE Profit. I Hope I Have shed some Light on this delima, And Think of the MONEY you Can SAVE on TWICE the Longevity with 1 Clutch Job. Again I hope this Helps you, And If you FEEL that I Deserve A GOOD RATING Please do so ,as Most folks get there Answers and GO without Rating US LoL Thank You !

STOP Downshifting!
STOP Forcing gear change, watch the Engine RPMs and shift accordingly, HOMBRE's are as good as the operator's. Hombre's have dash shift indicators which illuminate when it's time to 'shift' and not before, This has nothing to do with gear oil either. If you've drained it, you best fill it immediately and make sure it's ample level. The heat these gears sustain in is inceredible, cooling lubricant is essential. Here's a tip, while engine is OFF, ignition key is engaged, dash is illuminated, depress clutch peddle, keep it depressed and walk through all gears with gear shift, shifting should be smooth and unpeded, including reverse. If resistance is experienced at one or more gears, gear alignment is off as a result of inproper gear shifting while driving. Continue to drive vehicle just shift slower until clutch needs replacement then do an entire drive shaft upgrade. It worth it, it's great truck.