Q: So I found out mt husband used castrol high mileage 5-w30 oil on my gmc terrain sle 2013 and im panicking. Is that oil okay?

The high mileage oil brand, and the specific viscosity rating (5W-30), that your husband bought for you is perfectly acceptable and you needn’t worry at all. High mileage oil simply claims to have "extra" engine oil seal conditioners and perhaps added detergent, in comparison to "regular" oil, but you needn’t worry as these extra "additives" will NOT have any deleterious effect whatsoever in your engine. As to whether so-called high mileage oils render any benefits, though, I am not aware of any peer reviewed, published scientific papers proving a benefit from high mileage oils. Again, such oils will NOT harm your engine but a closely related question is simply, "is there a benefit, particularly in view of the fact that the price of the "high mileage" oil is higher than regular oil?". Until publication of such proof appears, from the standpoint of the public these so-called "high mileage" oils are a waste of money. If you have further questions or concerns, do not hesitate to re-contact YourMechanic because we want you to make the most of your repair dollars and help you to get the best possible results at the lowest possible cost.