I use 10w40 year around in my '88 Escort which is spec 5w30. I’ve been using 10w40 in it ever since I bought it with 146K miles on it except the last oil change when I used some 20w50 I had in the garage. My brother bought the car new and either ran 10w30 or 10w40 until I bought it, I’m not sure which. It currently has 517K miles on it. I did have to remove the oil pan a few months ago and clean the oil pick up screen on the oil pump (first time in over 500K miles), but I think most of the sludge problem was due to driving it on dusty construction sites for the first 12 years 350K miles of it’s life than the use of 10w40. The 10w30 and the 10w40 will both have the same lubrication at start up, because they are acting as a 10 weight oil when cold, the only difference is in warmer temperatures the operating weight will be 40 weight instead of 30 weight. With the mileage and age of your engine, I wouldn’t expect any negative affects from the 10w40. I maintain 7 cars and a motorcycle in my family and use 10w40 in all of them. My newest car is an '02 Ford Escort that is spec 5w20 it has about 118K miles and uses about a half quart of oil between changes (5K miles). For what it’s worth I’d have taken the advice of the son. Most likely if your oil pressure light was coming on it was at least 2 quarts low on oil. I’ve never seen a car where the light came on at only 1 quart low.