2000 Honda Civic with a JDM swap. Probably 150K miles on the engine. A guy doing a radiator replacement offered to change the oil. The engine is 5w30, but he put 10w40 in it saying that it was better for high mileage engines and it's what he puts in all of his racing Civics. My car is no street racer. Ever since the oil change, cold startups have been really sluggish, and I've had to crank twice a few times. The initial turnover after cranking slowly eases into a weak idle, and then after a few more seconds what is more or less normal operation.

Should I get another change and go back to the 5w30? Or is there some other problem with the engine that it should be able to handle the 10w40 better?

 

I wouldn't run anything that thick... 5W30 or 10W30 is the right call for a Honda street car with a factory engine.