THIS MAY SEEM A LITTLE SIMPLE, BUT IN MY 22 YEARS OF RUNNING, ALL OVER THE COUNTRY, THE SMALL GUYS DO NOT CHANGE THE OIL. YHEY CHANGE FILTERS, AND ADD OIL, TO REPLACE WHAT LEAKS, OR BURNS OFF. THE ONLY TIME WE CHANGED OIL (ON MORE THAN ONE SHORTLINE) WAS TO PERFORM A MAJOR MECHANICAL OPERATION, LIKE REPLACING A POWER ASSEMBLY, CRANKSHAFT, ETC. ON CONRAIL, THE SOO/CP AND ON THE UP, WE HAD OIL ANALYSIS LABS LOCATED IN MOST OF THE LARGER DIESEL SHOPS, THAT WOULD PULL "SPECIMENS" FROM ENGINES, AND WOULD CHANGE OIL, AS PER DIRECTIONS, AND CRITERIA, LISTED IN THE LAST ANSWER, TO YOUR QUESTION. FOR THE SMALLER GUYS, IT IS A MATTER OF "ECONOMICS", AS THOSE 250-400 GALLONS OF OIL CAN BE COSTLY, AS WELL AS FINDING A WAY TO DISPOSE OF IT. REGARDS, DAVE