Could 10w40 cause overheating
 

A buddy of mine with a 1996 acura tl has started having some minor overheating problems recently I change his thermostat a month or so ago and that is still good and his coolant is fine he recently did an oil change for himself and said he used 10w 40 since then he has been having these problems could it be the oil or is it probably somthing else

I doubt it's the oil but your buddy shouldn't be running such a thick oil when Hondas require 5W-20. All he's doing is killing his mileage w/ that thick oil. Have you checked the radiator? It could have obstructions by bent fins or mineral build up on the inside. Also, when is it heating up, at highway speeds or stop-and-go? Might want to check the fan as well if it does this in stop-and-go traffic.

I agree with bassman on the oil. Did you put the same temp thermostat back in as you out? How minor is the overheating, sitting in traffic or running down the highway? Now with the cooler weather coming on I would change it back to 5-20 for easlier starts and getting the oil to the top of the motor quicker.

its not the oil. check the cooling system.....

1.fan
2.fan switch
3.thermostat
4.radiator

hondas/acura are good cars but the stock platic radiator gets old, and needs to be replaced. the car is 12 years old.