Thin oil is for newer cars. thin flows and gets into every spot in your engine. THICK oil is for older engines, sticks better and gives more lubrication, but it is bad on new engines because it kinda weights them down and slightly reduces HIGH RPM power, not notisable thoug.

The best oil to run is actually 5w30 synthetic, with 90,000miles you would require 10w30 of normal oil but with synthetic it would be 5W30 which is the best choice. Quake and mobile1 are some of the best brands.
Also a Purulator Pure One filter is a good chice since it has the backflow guard, preventing the oil to flow out of it while you car is off, therefore minimizing dry starts. this is important on high milage cars that witch you dont know how the previous owner maintained it.