1998 GMC Sierra 1500 Low Idle, then dies

I drove my GMC Sierra (1998) round trip two days ago about 250 miles,
which is more than usual  As I got off the interstate and was stopping at the
stop sign, I noticed it wanted to idle low and was afraid it was going to die.  I
put it in neutral to keep it revved.  I kept it revved each time I had to slow
down significantly until I arrived at home.  Once at home, I stopped the truck
and it died.  I did notice some oil under the truck, which is the first time I've
noticed it.  After checking the oil, it shows more needed to be added.  I
purchased the high mileage Castrol and put some in yesterday but it
continues to idle low.  Please give me suggestions on what I need to do.  
Thanks in advance.