07 Yaris Sedan Manual Tranny Problems

[fiia]

Originally Posted by 1stToyota

And +1 on the Amsoil; made a noticeable difference in mine. Tried GL-5 Lubro-Moly 75W-90 Synthetic Gear Oil prior to the Amsoil; the Lubro-Moly made no difference at all over the factory fill.

For the 75w-90 oils, can you mix these with the standard stuff Toyota puts in (10w40) or do you have to completely drain that out first?
also, at what mileage intervals is it recommended to use these?
I have about 15k but I often drive through heavy traffic on the highway and sometimes experience similar shifting problems, so I was looking to use redline. Any thoughts?