BMW Repair: 90 535i Oil & Eng Combustion & coolant ?, castrol oil, engine combustion

Question
Hi.I have 90 535i-running super strong(I purchased used 9-2011-I tuned up removed the synthetic oil and replaced and put 20/50 reg castrol oil. The instrument cluster lights are out...So I am not sure of exact miles other than I was told about 140K
when I start the car from cold.the oil & engine combustion light are on. when car warms up the lights go out and never come on again. I do have to add oil periodically. So a BMW Independent Mechanic stated I need an Oil pump since I having to add oil twice since the oil change. And as long as the car is not sounding like a DIESEL, the burning of the oil may just be triggering the engine combustion light to come on..DOES this sound correct?  Please advise
Do you think that this could just be the Oil sending unit instead of an oil pump?
If I need to add coolant to the car..can I ad the BMW blue coolant, if it already has the generic green fluid -which I am assuming it does.but I am not sure. Will adding the blue hurt the car if the green fluid is still in the car?
Your help will be greatly appreciated
Thank You
Sondra

Answer
Changing from synthetic to an oil like Castrol isn't too much of an issue assuming the oil wasn't always synthetic.The consumption maybe because of the oil change.You need to examine the plugs and seals/gaskets. Any leaks will be found there.A compression test and a cylinder leakdown test will determine how well the rings and valves are sealing.If the valves are adjustable ,Adjust them.Coolant probably isnt blue so the green will be fine.If it is blue it maybe time for a change as it is 22 years old and factory recommends every two years.
As for the oil pump.I would check oil pressure.The pump only gives flow of oil. The pressure is the clearance in between the bearings and crank. The senders (this is an older car) maybe bad.Alot cheaper than a pump. Check the oil pressure with a gauge.