Oldsmobile/Buick Repair: Oil Type and Coolant Level, radiator coolant, coolant tank

Question
Hello Billy,
question #1:
 I've been using Castrol "High Mileage" motor oil in my 1989 Astro van, it seems as if the oil stays clearer much longer than when using the old Castrol 10W40.
 A guy told me it might be that there are less detergents in the "High Mileage" oil, and it just wasn't cleaning and dispersing contaminants into the oil as the 10W40 had been doing.
 If this is true, wouldn't that mean that the "High Mileage" oil is not keeping the engine clean as the 10W40, and I might need to switch back to the old type oil?
question #2:
 When I drive out on the interstate at 65 to 70 m.p.h. for about a week, the radiator coolant level drops down to an inch or so below the lowest part of the fill neck. The temp. gauge always appears normal and I have never seen any indication of steam or boilover. When I drive slower, around 60 or below,(on the interstate or in town), the level appears to stay normal. Can you tell me what might cause this?

Thanks a lot for any advise!
Mike Eidson  

Answer
hi ,mike. your first question no high mileage oil is just what ti says ,and yes has diff additives in it for high mileage engines.and the oil staying clear longer is a good thing.this means it is breaking down less then the other.the cleaner the better.#2 your coolant tank always adds or subtracts antifreeze as the temp demands it so you will see a diff from time to time.it should be checked cold and filled cold so if reading cold is normal it is fine do not over fill.hope this helps please take a min of your time to rate my help ty billymac