Five thousand mile oil change intervals would be acceptable for a new, clean and efficient engine. As it gets miles on it and develops a condition of "blow-by", the oil gets diluted with contaminants. Some of the larger sludge and varnish will get caught in the oil filter. Some will break down to microscopic in size and pass through the porous filter medea of the oil filter. The larger pieces restrict oil flow and the small particles change the lubricating properties of the oil. The restricted filter will slow lubrication getting to the top of a cold engine. That is when most wear takes place. Over time, the engine does not run smooth and looses efficiency. Eventually requiring repairs to get thru inspection. We can talk more about how the inefficiencies cause oil to burn and pass thru, and eventually damaging the oxygen sensors and catalytic converters. Maintenance is cheaper than repairs. I change my synthetic oil every 4 thousand miles. Conventional oil should be changed at 3,000 mile intervals. Get it out when it starts to change, not when it is used up.
Walko Automotive Service, LLC