mobile 1 high mileage 5w30
Ful synthetic oil is the best oil to use, it wont hurt your car with 20 miles or 200,000 miles.
Blown head gasket
About 4500 miles for car/truck over 150k
There is no way for grease to be in your oil for your expedition. There are no "grease-able" part on your engine or transmission for that matter. You are probably seeing either gummed oil or perhaps you are having a head gasket problem that is allowing water into your oil. This would make a mixture looking slightly like mayonase in your oil pan and engine. With as many miles as you are describing in your car, it is most likely the case that there is just some build up of sludge from inferior oil breaking down and 'sludging up'
After 3,000 miles you should have your oil changed and I believe after the first 1,000 miles on a new car it should be changed. If you have a lot of miles on your vehicle it might start burning oil.
It is best to change the oil after every 6000 miles. The most important thing is to follow the recommendations for your car.
5000 miles
When it is cold and on a level pavement One week after changing it, which should be every 3000 miles.
If your car has 100.000 miles on the engine,it's best to use 50 weight racing oil. My 90 lesabre has 293000 miles I use 10w30. Thicker oil will not flow to the bearings as quick and is not recommended by GM.
You should change engine oil in car or truck every 5,000 miles.
The best motor oil for you car depends on the car. Many old cars can only handle regular motor oil.
there is no universal best oil for a used car. The oil required for your car is specified in the owner's manual, or you can do a google query on the make and model.