I have a 1989 Chev pickup. Two wheel drive, 6 cylinder.

We replaced the engine with a used one ( out of good wreck) about 3 years ago.

I have keep it tuned up and it runs great, but anytime I get over 45 or 50 miles per hour for more than a couple minutes, the service engine soon light comes on, and will not go off till I turn the engine off. It does not do this at lower speeds, I can drive it around town 30 and no light.

It does not use any oil, no water lead, it does leak a little brake fluid around the master cylinder ( but not much,) I add a little about once or twice a month.

I have had it to a local garage ( not dealer) several times for tune ups, and general checks, but they can't find anything wrong, other than the light does come on.

Any ideas where to look ?

A clog in an oil line, it's probably not getting enough oil at the higher speeds cause the oil lines have a bit of buildup but are not totally clogged, that or the alternator is not producing enough power at the higher speeds, but this seems less likely as your battery would have died by now from not getting charged back up while driving, I would start by using an oil line cleaner.

Forgot to add, same for the fuel line, add fuel line cleaner, if fuel and oil line cleaners don't do anything have them check the power output of the alternator at higher revs, not at idle.

Ok, I guess we find out what area I have no expert status in at all, cars.

But oil line cleaner ( have not heard of that) is that something you can go into auto zone and find?

I don't actually know, my background is aircraft maintenance and I know we had oil additives that would clean the lines and did this periodically as preventative maintenance.

I'm sure any car care place could recommend one for cars, I took a quick look online and found this:

OIL SYSTEM CLEANER