oil degradation chart

For most mineral oils, a general rule of thumb is that the rate of oxidation doubles for every 18°F rise in temperature above 165°F.

Oxidation is what destroys the lubricating abilities of your oil and turns it to sludge.

To explore more than you ever thoughtwanted to know about motor oil, go to Bob is the Oil Guy.

This chart clearly shows the effect that high temperatures have on conventional oil. Full synthetic oils begin to break down at temps about 50 degrees higher than what is shown here.

You should keep in mind that if you have an oil temperature gauge it probably takes its reading at the tank. That's fine, as long as you realize that the hottest place the oil gets to is the collection gallery over the rear cylinder - and the one in the front cylinder is only slightly better. I had a sensor intalled in the left rear of the rear head on a mildly-built 80" motor once upon a time (milled heads, medium cam, Keihin CV carb & Supertrapp.) It showed temperatures 25-30 degrees higher than an infrared gun indicated in the tank.

When oil cools off it does NOT recover. Once it has hit those high temperatures for more than a minute or two you might as well consider it cooked; I do, anyway. I have never seen a test that finds how long it must stay there to be damaged, but I always figure if it's there long enough for me to notice, it's time to change the oil as soon as I can. Oil is cheap; engines cost a bunch.

A lot of riders say they don't want an oil temp gauge 'cause it's just one more source of worry. I say, fine for them. I'd rather know what's happening in my engine than to cross my fingers and hope for the best. Believe me, if I'm running across those Nevada passes in the summer and I see 300 degrees on my gauge, I'll be changing my oil ASAP.

Oil coolers are a good thing if you live in a hot climate. Again, there are guys who don't care for them. They say when you're running along you don't need one since the engine is designed to be air-cooled and when you are stopped they don't work because there's no airflow. Not quite, and the first part of that is something that'd be said only by someone who doesn't use an oil temp gauge. Let's take an 80mph run up Raton Pass in August, maybe even with the lady on board, and all her stuff. You will almost certainly see a 300 tank temperature, especially on a Twinkie motor, where Harley squirts oil onto the bottom of the piston crowns to keep them from melting. Keeping in mind what I said about the temps in the heads, your oil could easily be seeing 325 degrees, enough to destroy it's lubricating properties damn quick.

Keep this in mind about oil failure: running out of oil is one thing. It will probably result in an engine seizure and immediate severe wear on other parts. However, destroying your oil with temperature probably won't won't give you a catastrophic failure or seizure. No, what you'll wind up with is scuffed piston skirts and cylinder walls, worn rings, egged-out rocker bushings, prematurely worn cam bearings, etc, etc. The fact that your engine continues running while you use bad oil is not the same as taking care of your engine.

A well-designed oil cooler (Jagg and Lockhart, for example)will knock 20-25 degrees off that temperature and if you are using synthetic oil (like you should be) you have a reasonable margin of oil usefulness when the load and ambient temperatures are high.

How about when you are stopped? Does it work then? Well, yes, just not as well. Yeah, they need airflow to do their best work, but a properly-installed cooler will be tilted noticeably forward, which means that air is free to rise through it at a stop or slow speed, and thus carry away heat.

What goes into a well-designed oil cooler, besides oil, that is? It needs fins, to start with, and it needs them on the inside of the tubes as well as the outside (Jagg and Harley call these "turbulators.") Think about it. The outside fins radiate heat away but without fins on the inside to take heat out of the oil and give it to the fins outside you have no more total effective surface than is found on the inside of the just the tube. Make sense? And, of course, the more tubes and fins the better, so a 10-row cooler will have more effect than a six-row cooler. Friend, one of those fancy-looking billet coolers with fat fins on the outside will do you some good because it gives a little additional capacity, which is a good thing, but it's not really going to do much cooling for you.

To do its best work and have a long life, oil does need to get up to operating temperature, that is, somewhere between 180 and about 250 degrees. On a cool day, say, below 75 or so, it may not get there at all. So, you need a thermostat in the cooler circuit. The most convenient ones are built into the cooler itself, but others are built into the plumbing for the cooler. I won't go into that here; just, if you decide to get a cooler, make sure there's a thermostat involved.

Bottom line for me is, treat your oil kindly and it will be good to your engine. I use regular old Mobil 1 and have an oil analysis done every other time I change oil, usually around 6,000 miles. Every time, and I mean every time, the analysis report has shown that the oil is still good and the engine is showing no unusual signs of wear. That's a real comfort, I think. In fact, if I were on a long trip and my oil had 6k on it and I were still a thousand miles from home, I'd ride on it with it. Happy motoring! Pilgrim

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