With the 300ZX, we have a common issue with the syncros in 4th gear. At higher rpms, dropping into 4th can be very difficult and comes with a clunking sound. I didn't have any issue with this running redline, but after I switched to royal purple for a few months it was became very prevelent.
IMO, I'm more of a fan of Redline, but royal purple is better than most brands and its to find than Redline.
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