1968 40 big twin gear oil

[Dcerzan]

Looking to fill my newly rebuilt lower unit for the first time . need to know what gear oil to use

[Solarman]

If newly rebuilt, a good grade of fully synthetic lower unit gear oil suitable for marine engines, would be the best.
Or, you could run a good standard lower unit gear oil for 20-25 hrs as test, break-in period, before putting the good oil in?

Good luck.