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Sudden lugging problem - Synth oil? - bad gas?

I’ve got a '91 Saab 900 non-turbo that ran great, but suddenly started stuttering. Almost like lugging, but not exactly, and would happen as high as 1800 rpm but not higher, and would happen without especially high load. The problem seemed to begin when I did the last oil change and used a lightweight synth, which I haven’t used before on this car. I thought it might be ignition, but the wires are fairly new, and all activity above 1800 rpm is normal. This went on for a week or so, but seems to have gone away after I filled up yesterday. Bad gas seems unlikely though, since I had trouble through two tanks of Shell +, but yesterday I went to an X-brand station, and today had no trouble at all. What a mystery! I’m in Germany, if that makes a difference. Any ideas?

I doubt if light weight synthetic oil will make your car stutter.

Bad gas, a failing fuel pump or a partially clogged fuel filter could all be causing this. Your car is 18 years old and will need a good mechanic to finmd the root cause.

Good luck!