Hello, I've been using Mobil 1 Full Syn since I got my car with around 40k miles on it. They have a "High Mileage Oil" for cars with 75k+ miles. Is it a good idea to switch over to using this now, or continue using the standard Full Syn?

 

One of the main features of High Mileage Oils is a seal and gasket conditioner. If your Z doesn’t leak oil or burn oil. Just keep using what has been working for the time being. If your car doesn’t leak but has started to burn some oil, give the hi-mileage oil a try. If it is the valve seals responsible for the oil burning, this may fix it.

 

Don't do it! Use a PAO oil like m1 0w40 or redline.

Seal swelling is never a good idea.

 

I wouldn't. When I worked at Napa I used to recommend using high mileage oil for cars over 150k miles. Anything under as long as you are not having issues I would recommend using regular oil. In your case full synthetic.

 

1 Quart of lucas oil stabilizer and mobile 15000 mile full synthetic k&n oil filter. I ran this 30k miles in one of my cars before and the oil still wasn't dark brown after that time