560 SEL's recently developed miss

[ATLD]

1986 560SEL 110k mi.

Just recently I changed the 02 sensor, plugs, and all fluids on my fathers 560SEL. The oil used was winter weight 5w-40 full synthetic oil that my father had used in his other cars and wanted me to try on the Benz. I accepted skeptically. Typically, heavier weight dyno oil was used due to the voluminous oil capacity the car possesses and the cost of syn, but this was an experiment. The syn oil caused a significant leak from the front main that I will soon correct.

Anyway, the car was inspected a week later and given flying colors for everyhting and passed emissions. Not long after, the car developed a noticible missing at idle. Then, it wouldn't start on the first try, you'd have to crank it twice, or hold the key on start for a few seconds. Now, the car misses at all rpm's and at acceleration hot or cold. The symptom does go away sometimes and may start perfectly once in a while, but then recurs. There seems to be no trend that I can discern about when the car will miss and when it won’t.

I found out later, that after the inspection, my father added to a nearly full fuel tank, a can of Valvoline fuel injector cleaner.

Unfortunately, since all these things were done at nearly the same time, I am having difficulty diagnosing what may have happened to cause the missing and difficult cold starting. The problem has been progressively getting worse to the point where the car feels like multiple cylinders are missing over the full range of engine preformance.

I think the following are possible:

1) fuel additive caused some in-line contaminate to break free and clog either injector or fuel distributor (ugh)

2) thinner syn oil is blowing by valve seals (no blue smoke however) due to less viscosity and fouling plugs

3) bad 02 sensor (unlikely sine passed emissions)

4) plug wire(s) may have failed from being "disturbed" from plug changing

5) failed cold start injector

In the immediate future, I first plan to change the fuel filter, pull the plugs to check for evidence of burning or oil. Pull some injector lines and inspect in-line filters if so equipped. Check fuel pressure.

If I have to I will: replace the distributor, wires and (again) the plugs. Replace injectors (if testing shows that the pattern and opening pressure is incorrect) and if need be, try swapping out or rebuilding the fuel distribution unit (ugh).

Does anyone see anything obvious that I might have missed, or have a recommendation for what I should do first.

Many thanks,

ATLD