The only lubricant that we recommend to use with out gear sets is the NEO Synthetic 75w90HD or 75w90RHD gear oil. It's made in the USA and used in all forms of professional upper echelon motorsport. These are a GL5/GL6 API rated lubricants.
The Mazda manual specifies GL-4, I've heard that GL-5 contains sulfer and can cause premature wear or damage to the brass syncro's.
Is this info correct or have the syncros changed over the years and this is no longer an issue?
Our straight cut gears have minimal noise due to the precision and minimal backlash set by our production team. As mentioned previously in other posts, The advantage of the straight cut gears is the minimal stress placed on the transmission housing compared to the helical version. We have had many customers rip the end of the transmission housing apart when pushing extreme horse power.
As an option, we can also supply a housing reinforcement kit which slips into the housing and reinforces the bearing retainers/cups. Fitment of this kit requires precision machining of the transmission case on CNC equipment though.
This is presicely the reason to get strait cut gears. It also sounds like the noise level is at least slightly increased with your helical, so we all may as well go strait cut anyway. (dunno)
We usually supply the parts to replace the OEM items into your transmission. The fitment can usually be performed by any reputable transmission overhaul shop if they have experience overhauling the G-Series. The only technically challenging part is the need to grind the pinion shaft to a set OD for certain gears if you would like to fit the needle roller bearings. We make a large number of variants to suit different customer's requirements.
Can you do this machining for us before they are shipped?
Do you provide the bearings?