Redline makes excellent stuff, and I use their Water Wetter in the cooling system. When I took the performance driving class at Fontana last June, the guys on Team Lexus said they were running Redline in the engines of
the race cars.
However, having said all that, I would stick with Toyota trans fluid, which is specifically selected for the IS300. The Toyota fluid ATF Type T-IV, and the part number I have on my supply is 00279-000T4. I have four quarts on hand, purchased on eBay, but the local dealer price at
Cerritos Lexus is $3.79/quart.
I would stick with the factory stuff on this item unless you have a good reason to change.
I use redline in my Cobra and although I can't claim any seat of the pants improvement, I can attest to the fact that the Ford fluid looked a whole bunch worse after 10K miles than the Redline MTL looked after the next 25K. New tranny break-in could have had something to do with it, but I was convinced.
I assume that you are referring to an manual - I would stick with manufacturer recommendations for an auto.
I'm a big fan of Redline, BUT I read up on putting it in my IS and even the Redline tech support says DO NOT PUT IT IN THE IS 300! It uses a Type-IV fluid from Toyota that is unique and there is no Redline replacement. Ignore all this of course if you have a manual, and go buy some Redline.