He's got retrofitted pencil plugs, so unless the relay was switched out, the original relay won't be comparing plug 1 against the others. So, if you have the original relay, yes you can have bad plugs and the relay will not be giving an indication.
If it were me, I'd switch out to synthetic oil. The difference in cranking speed between dino and synthetic at those temperatures is substantial.

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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