About the car: 1978 240d, had for approx. 6 months. I have all the maintenance/service records and note that the compression has been low since about 18 years ago (96): #1:220, #2=220, #3=220, #4=250.. Have not tested since. It has only ~165k (odo works). So, I realize this one is always probably going to be on the edge of having enough compression to start. In its prime it had VERY low power, though could go ~70 on flat freeways, with enough time.

Problem/symptoms: won't start. It had some difficulty starting on recent colder mornings, needed trickle charge at battery after long attempts, and then finally reached the point where it has been for about 1 month. Cranks, but won't start. Using 2 ga. jumpers from my truck, no luck. Starter/battery seems tired after maybe 2-3 20 sec cranking attempts, even with jump. Starter is new within one year. Noticed oil was very low, but not dry (seems to be burning a qt every several hundred miles).

What I have done/checked:

--changed to synthetic oil, replaced filter
--new fuel filters (have not replaced tank) 2nd time I've changed in 6 mos. I also ran purge in separate tank twice to clean injectors before this no start problem began. It did seem to improve compression.
--bled hard lines, primed/filled secondary and primary.
--no leaks at pump
--cracked nuts at injectors and confirm fuel delivery while cranking
--vacume checks out, fuel shutoff not stuck
--tricked tablespoon of oil into each cylinder to seal, in case diesel dried it out and lower temps shrank up the seals.
--checked GPs (pulled 2, checked rest for voltage/draw)- all good. Relay good. oddly, resistance reads zero at all except one of them. Not sure why. Two of those with 0 ohms were pulled however, and are working fine. All of them measure approx 10v when drawing current, and all are getting 12V.
--running block heater before attempts
--Adjusted choke which had loosened, now tight.
--Turned crankshaft manually at PS pump, seems okay.
--Battery voltage okay, but seems a little sensitive to my attempts to start. I trickle charge it over night with 1.5Amps and charger says battery is good and charged after a day or so. Car cranks and sounds like its going to go the first attempt or two, but then... maybe 2 cranks per second. Slower and slower. Voltage is like 12.8 before starting.
--checked valves, which seemed a hair loose, if anything. None were too tight. Did not change (lack bent wrenches).
--won't even start with WD-40 sprayed into intake while the wife cranks.
--alternator belt seems loose, so I'll tighten.

This car is definitely hanging onto the low side of life, but the original owner (30yrs) took good care, and it suits us fine. The temp is like 40 here, so it isn't criminally low. Could it be solenoid, even though it is only a 1+ years old? Is the compression so low that it just won't start in the cold? Nozzles? What else should I try, what else can I check? Keep in mind that I'm definitely a novice, and low on time. Car is marooned in driveway.

I really appreciate any ideas, expertise. I've probably learned everything I know about maintaining this vehicle from this forum.