It is a 7.5L engine with 45K miles on it. I last changed it in the spring (about 5 months ago) with CAstrol GTX 10w-30 and since then put about 1,000 miles on it. Most of the trips have been short and a few longer ones. Is an oil change necessary? Does oil degrade with time? I have Chevron Delo 15w-40 which I plan to use next time.