1992 Chevy Beretta below

I will send a donation later when my new atm card arrives. Meanwhile, the "check engine soon" light comes on occasionally, especially on long downhill slopes. I have put in a nice new change of Quaker synthetic, (full), 4 quarts, and the dipstick always reads full. And clean, I can see it. When I used to use organic oil, I'd get the same CES light, so I put in up to a pint of STP, and eliminated the light, for a while, but then it returned, so I put in more STP, and it worked. Since the synthetic oil will last all year, and the CES light still comes on, I thought maybe you know something I should do to eliminate the CES. Something other than STP? I don't want to compromise that nice new $30 synthetic oil change. I will contribute later. Thanks, Dave.