Just FWIW a wee update, I changed the oil filter and top o-ring. I'm not really bothering with oil changes as I've added over a sump full of oil to it since I've been using it (at least 8x 670km/416 mile trips)..
Anyway the issue has improved slightly to the point I can't be bothered chasing it any more, the car has other issues to attend to.

(Also it was 20W-40 oil rather than 50...but only the 20 part counts when cold of course)

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1978 300D, 373,000km 617.912, 711.113 5 speed, 7.5mm superpump, HX30W turbo...many, many years in the making....
1977 280> 300D - 500,000km+ (to be sold...)
1984 240TD>300TD 121,000 miles, *gone*
1977 250 parts car
1988 Toyota Corona 2.0D *gone*
1975 FJ45>HJ45
1981 200>240D (to be sold...)
1999 Hyundai Lantra 1.6 *gone*
1980s Lansing Bagnall FOER 5.2 Forklift (the Mk2 engine hoist)
2001 Holden Rodeo 4JB1T 2WD