I'm pretty sure 79's came with loop plugs but the problem is that there are retrofit pencil plugs for the car so they could be either. You need to look at the plugs and see if they are connected using heavy squiggly wires or not. There's a tutorial on Diesel Giant on pencil plugs but you should not follow any of those procedures until you've determined if they are loop or pencil plugs. Posting a pictures of the plugs here would get a quick answer.
There is no easy answer to the oil question. Just use diesel rated oils.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13