Getrag has stated that using Redline fluid or anything other than Getrag-Toyota V160 fluid will cause seals within the trans to shrink or expand at different rates. I know this for a fact; I used Redline D4 ATF and my tranny started to leak from the shift shaft seal. Within a week of replacing the fluid with the V160 fluid, the leak completely stopped....I was lucky.
The fluid is available from Toyota. It is called V160. The parts guys always give you the wrong fluid, because they have never seen it before... but it is in the Toyota part guide and stocked in Toyota's wherehouse. It is about $45-49/litre. You must buy two litres, because the MKIV takes 1.8 litres. Even though this is expensive, it is cheaper than a $5,000 trans. Toyota lists the trans as containing non-replaceable parts. So if you ruin it, i'ts $5,000 for a new one!
Here is the link to the thread on this topic. It's actually in this forum and on this page. Buy the Toyota V-160 gear oil. Get it from Jay Marks Toyota in Texas (look up the # ) and ask for Jeff Watson in the parts dept. He will give Supra guys a good discount. Unless you have a good relationship with a local dealer the fluid can cost around $45 a litre and you will need 2 litre cans to do the job.
For the diff you can use Redline heavy duty shockproof gear oil. I don't remember the grade to use. Maybe someone will chime in with the answer. The diff is not as finicky as the transmission, you just need to buy the proper grade and type of oil. ONLY use the Toyota V-160 fluid for the tranny. Unless you feel like experimenting with a $5,000 part.
I was beaten to the punch! No matter, we said (weirdly enough) almost the exact same thing.