When to change the oil...
I base my oil changes off of many variables.
The first and foremost important variable is the operating temp of the oil. Now, to get the actual operating temp of the oil you must pick-up the oil temp remotely from the engine so the radiant heat from the air cooled engine is not giving you false readings. It is also dependant on the oil you are using and the ability for it to work in our extreme application.
I personally and recommend to my customers that use my oil cooling system that they can go 6-8 oval races between oil changes. I use Valvoline VR1 20w-50 racing oil. I use a Wix 51069 oil filter and two oil coolers.
My oil cooling system runs between 220-240* in the heat of the summer asphalt oval track racing. I have several guys that use my oil coolers system because it has been proven over the years to control heat and they don't run an oil gauge to “save weight”.
For my roadracing program we change the oil 2-3 times a year. Once over the winter before the first race, somewhere halfway through the season and then maybe on more time right before the World Finals, but since the oil never get above 200* the oil stays fresh for a long time. Even when the oil is drained the color still looks good and it is not burned up.
Now, if a car gets the oil up into the 260* range, the oil must be changed no matter how many races. The oil gets burned at 260* and above and it is obvious when the oil gets there it just keeps on climbing and will not cool down.
I do not change the filter unless I am, changing the oil; the automotive filter we use is big enough that we shouldn't really have to change it at all. It is at least two times bigger than the bike filter is.
I cannot answer about dirt racing, but if it was me, I would use the same criteria.
My recommendation would be to monitor actual oil temps and change the oil accordingly.
And as one of my customers tells me all of the time..."oil is cheap, I change it more often just because it makes me feel better..." The inside of his engines don't look any different than mine...
Randy - RPM