If your 80 is like my 79, the PCV vents into the air filter to turbo U tube. I'd check to make sure that the constant high rpm's of highway driving is not pushing oil past the valve cover baffle into that U-tube and it's leaking out somewhere on to the passenger side of the engine.
I'd clean off the passenger side of the engine at the car wash, go for a highway drive of 10 miles or so and then see if I could find the source of the oil. On the passenger side I think it's limited to either the turbo oil feed/drain system or the PCV hose, if it's not the valve cover.

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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