Oil viscosity question.

I have a 1999 nissan altima with 112k on it. at 75k the car was switched over to Mobil 1 synthetic 10w-30. I just bought the car from my dad and before it was mine never really paid attention to the oil grade he used in it. I was looking through the manual the other day and i came across the section that had the information about what type of oil to use. It said that 5w30 is preferred for all temperatures but 10w30 could be used if the outside temperature was going to be above 0 deg F. I asked my dad if he had seen this and known that 5w30 should have been used. He said that he did but he used 10w30 because the car had so many miles on it when it was switched over. My question is could anything have been damaged by it? The ambient temperature never got below 0 deg F one would think not but my buddy said that the use of 10w30 when 5w30 is prefered could have increased bearing clearances and possibly caused some sludge buildup. Should I go back to a 5w30 or stick with the 10w30? will i have any problems if i go back to a 5w30? Thanks!!!!

IMO no damage was done due to the use of 10W 30. Your buddy is right in theory about 10w 30 being preferable in a well - used engine.
In practice, though, it depends on the amount of engine wear.

Often manufacturers recommend 5w30 or thinner oils due to the tighter clearances of modern engines and the slightly better fuel economy that 5w 30 will give you.

I would suggest to use 5w 30 in the winter months and 10w 30 in the summer.

ditto^^^