Datsun 1200 can use many engine oils including these modern blends:
* 10W-30 * 10W-40 * 20W-50
Factory Datsun 1200 Specifications
Reference: 1972 USA Owners manual original Specification:
* Multigrade: SD MIL-L-2104B or * Monograde: SD MIL-L-2104B
Nissan A-Series
The 20Ws are good down to freezing, per the owner's manual chart, while 5Ws and 10Ws are good for colder climates.
And also they specified some single weights, but those have a limited temperature range.