I burned used motor and trans. oil in my 240 for the last 2 years.
I made a filter that cleaned the oil after about 48 hours.
I had at times in the summer as much as 50% used oil. During this time I had no smoke after warm up and only slight smoke on starting. I put at least 30'000miles during this time on used oil. Never had any ill effects on engine or injection system. That car is still running today with 330'000 on original engine and pump.

__________________
Ricali

03 C240 4matic wagon
95 300E 234,000
7 prior 240;s 5 still going
81 300sd gone
65 230sl gone
49 Studebaker Champion
90BMW convert.167,000
60 Dodge D-100