All right, problems with this car are coming up constantly.

I just found out my Civic 91 DX has been driving on mineral based 15W40 oil for at least 6 years now. My service manual suggests a full synthetic 5W30, and I read somewhere that you should do some kind of "flushing" of the whole engine oil system before you switch from mineral based to full synthetic oil. Does anyone know anything about this? Or do I just drain the mineral oil and pour in synthetic?

Thanks,

Kresho

Good advice on things like that are hard to come by. Some people always want to take the easiest way out. Others will go with any extra procedure, no matter how marginal the benefit. I would just change it. The synthetic likely will clean out some old deposits, maybe even ones keeping the oil from leaking. You might go with a shorter than normal change interval to remove some of the excess material the oil may pick up. The 5W-30 should make the car start easier when it is cold, and give better lubrication before the engine is warm. Oil pumps have bypasses on them that can let the oil return to the pan rather than flow to the engine when the oil is too thick. There is also a bypass that lets too thick of oil bypass the filter too. Unless where you live is fairly warm year around, the 5W-30 should be much better for your car. If the engine has a lot of wear, you may still need a thicker oil. Watch the oil pressure with the new oil. Even if it is lower, if it is as high as it should be, you may have more filtered oil flowing to the engine.

Hm...

The weather here is like this: at least three months a year (summer) we have around 95 deg fahr, and at least 2 months (winter) we have around 27 deg fahr. So kind of big oscillations, yeah.

I am interested in what you said about easier starting. The car cranks at least 5-6 times before it starts now in the winter, and I checked battery (idling, charging... ) and connections and they work great (so does the alternator). I guess synthetic oil should help about that.

Now about that 1 cylinder of mine that has lower compression... if I put in synthetic, would it cause engine to burn more oil? Since the last change, when Castrol 15W40 was put in, engine doesn't lose oil no more.

Kresho

A heavier oil can reduce mechanical problems and extend the use of an older engine without expensive repairs. If you are going to a full synthetic you might look a 5W-40 which would give you easier cold starts, but maybe keep the oil usage down too. As the synthetic cleans up old deposits in the engine, it is possible it might free up sticking rings reducing oil usage and increasing the compression. It will not fix worn or broken rings or burned valves. Look for ester content over PAO base.