GMC Repair: Oil Type and Coolant Level, radiator coolant, coolant level

Question
Hello Peter,
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
hello mike
when driveing faser the water pump is workimg harder and will drop the level some this is what is happend to u dont worry it is ok .

oil

Go back to the oil you were useing if you are useing oil then go to the high milage oil . it is for only cars that burn oiul and what have problems and also no power pete