I'm looking at buying my first semi truck. It's a 1995 freightliner with a cummins n14 that hold 38 quarts of oil. The truck comes with maintance records from day one. They changed the oil every 10,000 miles with Shell Rotella T 15W40 Motor Oil. So my question is this winter I will be parking the truck were we have no place to plug it in. Should I keep 15w40 or change to something else? Will it hurt that I'm not pluging it in? I live near St.louis Mo so the average winters are around 21* for a low and last around 3 months.