Is it turbo or n/a? Year model? Does it get cold where you live?
I don't believe the 4 cylinders in those cars are too picky with oil weights. My parents 240gl, which I believe has the same engine as the 740, has been run on plain jane 20w50 for the past 150,000km, it's up to 480,000 atm. Still runs like a top and burns no oil although the climate in QLD is a bit warmer and it's had its fair share of wear so a grade thicker is expected. 15w40 is usually conventional oil and 10w40 is semi-synthetic so the former is usually much cheaper, I don't think it matters all that much but I could be wrong.
There's your official specs from the manual if it's been lost.