But my concern is the GL-4 vs GL-5 grades. Toyota wants GL-4 since it doesn't ruin brass syncros. The only GL-4 I know is redline.
You may know something about the chemical differences between GL4 and GL5. You may also know exactly what the specific metallurgy and alloy formulation is on the synchros that are in your transmission.
I do know that my transmission shop would only install Toyota factory synchos in my 5-speed transmissions, and told me that some aftermarket synchros have incorrect alloy that cause a problem with proper co-efficient of friction and lock-up. They didnít mention anything about gear oil compatibility.
When I spoke to Redline technical department back in 1995, Redline did recommend MTL which is GL-4 for my transmission. MT-90 is also GL-4.
I believe there is sufficient information now that the early formulations of gear oils with reactive sulfurs, that caused a problem with corrosion in copper alloys, are no longer used in modern gear oil formulations.
I donít believe ExxonMobil, the largest USA oil producer, is going to sell a synthetic gear oil that will destroy synchronizer gears in a massive number vehicles with manual transmissions still being driven world-wide.
Hereís an interesting short video on the comparison:https://www.youtube.com/watch?v=XHwULF-EdtQ
To sleep well at night, using a high quality GL-4 oil in our transmissions wonít hurt.
Thatís just my opinion Ė it may be worthless.