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 10Ws are good down to freezing, per the owner's manual chart, while 5Ws and 10Ws are good for colder climates.
Recommended * 5W-40 * 20W-50 * 10W-40 * 10W-30 * 5W-30
Nissan also specified some single weights, but those have a limited temperature range.