Two different engine oils inside the engine
1991 Volkswagen Golf Cabriolet (first generation), 89 902 km (55 862 mi), 3 - speed automatic; 1.8 liter gasoline.
My colleague at work bought this car in February and uses it to commute to work . It is a one owner; garage-kept: rust-free automobile.
Car's been running on 10W-40.
My colleague was driving on the highway when the oil light started blinking. Unfortunately, she didn't have the right oil specification with her and poured xxx-70 into the engine (about 0.5 l / 16.9 fluid ounces) just to get home to pour the right viscosity in.
Car runs good; no check engine light or oil light on the dash. Could this have caused problems? Will they appear later? Thanks. She hasn't had the oil changed since. (this happened last Thursday).
She made it home with no catastrophe. But now, change it ASAP to the correct viscosity to avoid catastrophe.
That car is old enough, it may not hurt it at all. Read in the owner's manual to find the actual oil it's supposed to use. If the engine is an old design, it may tolerate a wide range of oils, particularly heavier weights. The owner's manual will have a chart of temperatures and weights. I put 3 quarts of 10W-40 and a single quart of 20W-50 in my 1979 Pontiac in the summertime to help slow down a rear main leak. The owners manual specifies a range of oils and temperatures, and those two are both compatible in my area in the summertime.
If the above doesn't apply, it won't hurt the engine if it's a lighter spec oil and she changes it to the right spec quickly, as in the next couple days. If she drives it a significant distance, it'll wear the engine out prematurely. More modern engines require specific weights, or they will wear out unnecessarily.
There should be no permanent damage, but drain out the incorrect oil.