For me, changing the oil on one of my cars is a spiritual experience. My frame of mind when performing that task is always peaceful. My heart soars with joy when I work on cars, restoration or maintenance. While I draw on my engineering knowledge when performing automobile work and generally do a meticulous job, I can’t put into words why it brings me so much joy. I could pay someone else to change the oil, although the time spent going somewhere and waiting would add to the expense. Then there is the possibility of incompetent work, which I encounter more often then not no matter where I take a car when I pay to have work done.
The odometer on my 1994 Volvo 940 Turbo is not working, so I guesstimate the mileage between oil changes based on my work commutes and other trips. Although this is far from accurate, I have a rough idea. Ideally, I’d change the oil every 3000 miles, regardless of what the manufacturer recommends. “Cheap insurance” many have said, which is true. With adequate lubrication, wear on mechanical parts will be minimal, running almost forever without need of repair. With inadequate lubrication, mechanical parts will wear out quickly.
I changed my oil today, knowing that I’ve exceeded the desired mileage interval. Although the manufacturer recommends a 5000 mile interval for this turbo engine (a Third Generation B-230F), that is pushing it. I’m estimating that unfortunately I’ve gone somewhere over 4000 miles, but not over 5000 miles, since the last change. In general, 3500 miles is about as far as I will go between changes without feeling uncomfortable. I’ve seen a chart of oil viscosity versus miles. What happens is that the oil maintains adequate viscosity and protection up until a certain point, then instead of gradually losing viscosity, the viscosity simply drops off, like a stone falling from a cliff. There is a discontinuity on the curve, where a moment before there was adequate viscosity for engine protection, then then next moment there is almost none. The outside temperature when I went to change the oil was in the mid 40s F. I let the car engine idle for ten minutes to let the oil warm up. After changing the oil and filter, I carefully poured the used oil into a container to carry it to the recycle point. As I was pouring it, it poured like water–no noticeable viscosity. Yuck. That’s oil that has been in the engine too long and is providing very little protection. I’ll be sure not to let this happen again. Well designed engines will go millions of miles without trouble, but not unless the oil is kept in good condition, i.e. with adequate viscosity.