Toyota Repair: 1999 4 Runner Limited, fuel system treatment, oil additives

Question
Hello, I have a 99 4 Runner limited. 6 cyl.auto.4wd.  106k.  Bought from my brother in-law who bought it new with 17 miles.  He followed the scheduled maintenance all done by a toyota dealer.  Right before I purchased it he had the 90k service done.  It's amazing when you look in the maintenance book all the red toyota dealer stamps and every service done.  It starts up all the time but seems like it has a hard time.  I really don't know what could be wrong with it, it was fully checked out by the dealer.  ? :-(  Oh and by the way do you believe in any of those fuel additives such as fuel system treatment or injector cleaner etc. and oil additives like restore?  And I have always gotten conflicting information on whether I should spend the extra $$ on Mobil 1.  Thanks :-)

Answer
You don't give me enough detail as to what happens when "it has a hard time" starting, it could be either a battery failure or a starter problem or something else, please try to describe to me exactly what happens when you try to start it and if it makes any noises, like clicking etc.
I really don't have much of an opinion on the additives or the use of mobil one oil, if you can afford the extra expense of mobil 1 use it, it's your choice.