First, regardless of which oil weight you use, you need to ensure that it's oil engineered specifically for diesel engines - do not oil for gasoline engines, which is not engineered for the higher temperatures produced by diesel engines. In North America, that would be any API CJ-4 rated diesel oil.
15W-40 tends to be used as a "default" in many places... to go up to a 30W would typically require an environment with extremely high temperatures. And, without knowing which engine it is, we have no way of knowing how tight the tolerances of that engine are... if it's an engine of extremely tight tolerances, you don't want to go with anything heavier than the OEM recommendation.
Don't know what diesel engine you have or the climate you live in. Follow the manufacturer's recommendation.
Prescribed in the owners' manual is SAE 20W-50. The nearest best grade to use is SAE 15W-40.
No you can not use gas engine oil in diesel engine oil because processing of diesel engine is different from gas engine oil. If you use this then it damage your vehicles.
NO, do not use diesel oil and a gasoline engine.
Use exactly what the manufacture recommends as listed in your owners manual. Using anything else will cause engine damage.
You can use any SAE certified engine oil in your Nissan. The engine oil should be a 10W-30 weight engine oil.
SAE 10w30
sae 30
"Diesel oil" or "Heavy oil" is another term for diesel fuel. Not to be confused with "engine oil for a diesel engine" which means lubricating oil specially formulated for use in diesel rather than petrol engines.
Is it a 2 stroke or 4 stroke engine, Gas or Diesel. I would suggest consulting your owners manual. On a 4 cycle engine use SAE 10w30 synthetic.
No, use exactly what Cummins recommends and nothing else.
SAE 10w30 or SAE 30w. I highly recommend you use Synthetic oil in this air cooled engine. Change the oil once a year.
You can use any oil that is for on-highway diesel use. Rotella, Royal Purple, etc. all have diesel engine oil.