Oil system losing prime?

[NZScott]

1977 280>300D, OM617.912 auto
20W-50, ambient temps about 0-20 *C.



Been a while since I've felt I've had to ask for thoughts on this forum...
Been using the '77 I've had for about a month now as a daily driver. For most of that time, it has had the long wait for oil pressure to build like the oil filter housing is draining out.
I replaced the 2x o-rings on the oil filter lid stem, and it helped a wee bit but didn't help as much as I'd hoped.
She's an old engine but still shouldn't have to fill the filter housing each start (I presume that's the issue).
Oil is literally cheapest I can get my hands on with a Cx rating...it burns a lot as I now do 8.5hr road trips semi-regularly now I've moved house. I realise going to 5w-40 etc will help but it's not the cause of my issue.


So how else is the filter housing draining?



There is no external leak from the lid big o-ring so I doubt air is getting in that way. As an aside on my '78 with turbo conversion I had to put a check valve in my turbo feed line which was tapped off the housing because it was letting air get in backwards via the turbo drain and causing the same delay post-start before pressure came up.
Cheers...

Forgot to mention. Idle oil pressure when at normal temp (85-90) is 1.5bar, same as my '78 (with 10W-40). Goes to 'bout 1-1.2bar when cooling off from 100+*C.