I've seen both used in different posts by different people. Is there any sort consensus as to which is best for the old 240 4 speeds?

Don't know if there is a consensus. Either should work, but the MT90 is specifically designed by Redline to use in manual transmissions that originally called for 90wt gear oil, like our Zs did. I'm using MT90 in mine, and have no problems. I use MTL in the other (non-Datsun) cars.