When Do I Start Using High Mileage Oil In My Car or Truck?

Why does my 2004 Suburban Transmission slip? How can I fix it? Why does my Ford Flex have Gas Pedal Lag? it's not safe. Why does my GMC go into 4x4 Low by itself? My Subaru has a banana in the tailpipe or at least it feels that way. When do I start using High Mileage Engine Oil and what is it? Why does my Chevy Scottsdale 454 Transmission slip? Why does my 2011 Ford F150 Trans slip? Lots of slipping transmission here. When was the last time you serviced your transmission?