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

Question
Hello Anthony,
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
Mike,
If the engine oil you're using appears to be working properly, than stick with it.  "High Mileage" engine oils usually have special seal conditioners to prevent internal oil leaks.  Most engine oils do have some sort of detergents.  If your engine was real dirty, than you would see evidence in your oil.  If you are using a high quality oil filter, that will also keep your engine clean.  If an engine oil enhancement sounds like a gimmick, it probably is.  If the oil and filter that you're using appears to be keeping your oil clean, it probably is.  Also, replace your air filter as recommended by your owners manual.  That will also help keep your engine clean.

I'm not exactly sure how you're seeing your coolant level while driving 65 mph, so I'm assuming your looking at your overflow tank after you've been driving and then parked.  As the coolant temperature increase, that increases the pressure in the coolant system, causing the radiator cap to lift up (compress it's spring).  When the cap is fully lifted, excess coolant will be pushed into you overflow tank.  This tank will appear to be full.  But if the coolant is not under as much pressure/heat, coolant will be sucked out of the overflow tank and back into the engine.

Since your car is over 15 years old, it's imperative to keep up regular maintenance.  It may be time for you to get a complete coolant flush of your system.  Along with the flush, you should also get all your hoses checked and replace any that look cracked or worn.  This is also the time to replace your radiator cap and your thermostat.  All these items are relatively inexpensive.  And is a lot less expesnive than a blown head gasket if your engine's coolant system fails.  
Good luck!