What can make a 1994 Buick LeSabre power down and stall while turning after oil change and valve cover gasket change engine started grinding when turning after a stop light then stalls.

Grinding stopped car would not restart right away but restarted shortly thereafter and engine oil was at overfill point. Engine then restarted and was driven about a mile with no problems except oil hi on the dipstick and drips on the floor of the garageI don't see any coolant in the oil but the car only has 110,033 miles on it and the work was done only 33 miles ago so the oil is a little dark to me for only having 33 miles on it and again no water bubbles or coolant appearing in the oil and I have 3 people that checked the oil the day after work was done and said that the oil was overfilled but of course the shop manager winston said that they have never done that kind of a rookie mistake in the 40 years they have been open.