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
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.
No, you cannot use ordinary car engine oil. Use a JASO MA or MA2 approved oil, or SAE SF or SG graded oil.
SAE 30 weight in the summer or SAE 10w30 if you use the engine in cool temperatures.
SAE 30W in the summer and SAE 10w30 in the winter. I recommend you use 100% synthetic oil in any air cooled engine.
Transmission oil Motorex, SAE 15W-50
No, you can not use engine oil in place of two cycle oil.
It depends on which engine you have. 4.6L engine: Use SAE 5W-20 engine oil. 4.0L engine: Use SAE 5W-30 engine oil. You can find all of this information in your Owner's Manual. If you don't have one, here is a PDF of the 2004 Explorer Owner Manual.
I am not sure what you are wanting to use the oil for. If it is for a lawn mower, most of them take either 10w30 or SAE 30. They are not the same oil. You will need to refer to your engine manufacturer for the oil required for your engine. Normally most Briggs and Stratton and Tecumseh engines use the SAE30 oil. While most Kohler engines use 10w30.