I used synthetic.
Manual Transmission & Transaxle Gear Lube 75W-90
I've had the car out again for several short drives and the gear clashing is definitely gone. Seems like Passon got some bad advice from Lucas about what type of oil to use. Based on my research and experience - do NOT use a GL5 oil in the tranny. GL5 oils have friction additives specifically meant for rear differentials. GL4 oils have a completely different additive package that are meant for manual tranny's.
Jim