This FAQ about motor oils is in two main sections. The first section, on motor oils, thier use, properties and limitations was authored by a member of the Antique Tractor mailing list. (Thus the many references to Tractors which you can change each time to "Motorcycle" if you wish. His name is Guy Burnham.
Also check out the Motor Oil FAQ, Part 2.
This may get a little long, but I want to share with you some of the things I learned in my locomotive days. I worked with the engineers for Sulzer, a Swiss manufacturer of diesel engines, with Mobil and with a couple of filter companies, one of which made most of the "house brand" filters including OEM filters for GM and Chrysler.
Oil performs several functions.
- Corrosion protection
- Contaminant transfer
What I learned is that there are numerous tradeoffs made in all these areas and it is useful to look at some of them.
Some of these areas are in conflict. You can count on regular, automotive oils to be a good compromise for use in modern automotive engines. If your use is otherwise, it helps to understand what they did.
DETERGENT/NONDETERGENT- Detergent additives are just surfactants which lower the surface tension and allow small particles to remain in suspension more easily. This is to transfer contaminates to the filter so they can be removed. You do not want deposits to form throughout the engine because that makes them hard to remove and insulates the passages so that the oil can't remove and equalize the heat. The base viscosity is increased somewhat by other additives to compenste for lower surface tension. Back when overhaul intervals were shorter, The deposits would get cleaned up periodicaly before they got too bad. I wouldn't use nondetergent in an engine with modern parts.
There is little problem with detergent oils unless they are subjected to very high temps or pressures or used in hydraulic systems.
High temp and pressure can convert some additives to shellac like compounds that are really hard to remove. I think the diesel rated versions are less prone to this but it's not a big deal in most applications. Also, there may be extra foaming if air can get introduced somewhere. I had trouble with my JD M with detergent oil in the lift system.
One problem you can have as mentioned by others is adding detergent oil to an engine that has a huge amount of deposits in it. As the detergent softens these deposits, there is a risk of a chunk coming loose and blocking something. The risk is real but then if you have this much stuff in there, you have a time bomb waiting to go off anyway because a big temperature swing can trigger the same thing. The right answer is to tear it down and clean it up. My answer is to run detergent oil, at moderate load, and change oil and filter frequently for 3 or 4 changes and hope for the best.
OIL GRADE- The numbers, 30 weight, 15-30 weight, etc. relate to the viscosity of the oil. Viscosity is a measure of the "shear strength" of a thin layer of oil. This is important because of the way a plain bearing works. It is not the oil pressure generated by the oil pump that keeps the crank separated from the crank bearings (for example). Rather, it is a local area of higher pressure generated by the shearing action of the parts moving relative to each other. This "hydro-dynamic bearing" layer is what resists the forces of pistons and so on. If the oil is too thick, this layer will not form fully or oil may not even get into the space to start with. Too thin and the layer will not be thick enough to prevent contact at the high spots.
For an engine used fairly little, when oil changes are infrequent, multi-viscosity oils are important so that lubrication is adequate in either winter or summer. However time, temp, and chemicals, break down the additives that control the viscosity in these oils so that it changes over time. One reason they recommend at least annual changes. If the tractor is only used in a narrow temperature range, then a single weight oil would last longer without change since it theoretically contains less additives. However, oil companies may use the same base stock and add stuff to get various single grades, so I wouldn't count on more stability over time.
CORROSION- An important function of modern oils is corrosion inhibiting. That is because there are so many different metals in a modern engine. If the Ph is too far out of whack, galvanic corrosion can eat things up. There is a microscopic variation of this called "fretting corrosion" that can get to parts that are in contact but move very little under quite a bit of pressure. That is what gives the odd stain patterns on shafts and surfaces that are together a long time. The Ph of the oil is really affected by the water it picks up because the water will react with gasses to form acids. That is why lots of short trips are bad, lots of blowby and gasses and lots of moisture that does not have time to be driven out.
Oil starts out a bit on the basic side and gets more acid over time. I think they assume that most cars get a fair number of short trips. This is a bit of a problem for a tractor (or motorcycle) that does not get used too much since a basic Ph will corrode aluminum and an acid will corrode zinc and iron. Ideally you could keep "half worn" oil all the time but you can't. I use this as a justification for not changing oil too frequently.
FILTRATION- This a place where a lot of compromises are made and specs are pretty vague. In general, filters are rated by the pore size in microns. However, this is not a absolute limit due to the construction of the filter. The rating indicates that the filter will trap 90% of particles above that size. What they don't tell you is what is the biggest particle size it will pass. Good quality (expensive) media with a 20 micron rating will pass some particles up to 45 microns. Fuel filters have about an 8 micron rating with a 25 micron max. The tradeoff here is flow rate. The higher the filter efficiency, the lower the flow. A good "trick" some companies use for marketing is to go to a smaller rating paper that has bigger max pore size to get back some flow rate. They advertize "We have 16 microns and they have only 20" but they don't tell you they will pass a bunch of 60 micron stuff that "they" would trap.
Interestingly, there is a standard "test dirt" you can buy for these tests that has a known distribution of particle sizes. Engines are pretty tolerant of particles up to about 30-35 microns so the small end of the scale is really not what you worry about. Most filters actually improve in efficiency with use. That is because the big holes get bridged over and the surface becomes more uniform. The best quality filters have a micron rating that is fairly high, say around 28 but is very uniform pore size. This paper improves pretty fast to around an 18 actual perfomance and stays that way a long time.
A bad filter on the other hand has small pores so it tests well when new but a wide variation in sizes. The small pores tend to plug over in use and the big holes open up with the added flow through them. This results in a worsening efficiency condition. A filter that plugs too fast can also rupture or cause the unit to "bypass" the filter so that there is no filtering at all.
Unfortunately, there is little you or I can tell about filters until it is too late. If you could get them, you could compare the manufacturers graph of efficiency, flow loss and max particle size over time (actually volume filtered). The best have a broad time in the middle range with high efficiency and low flow loss. The manufacturers have these, but I have never seen such graphs available at the retailer. The best bet is to buy good quality brand name filters. The highest price ones are probably no better and the lowest price ones may or not be the same. It isn't worth the $3 difference to find out. Never use fuel filters for lube oil, by the way. They will plug too fast and it is overkill. If a tractor is not used very much, say just for shows, I would consider changing oil each year and filter every other time to limit the amount of big particles getting through. This would not be true if you are "cleaning up" a dirty engine with detergent oil.
OIL TESTING - I should mention that a lot of truckers etc use a testing service to keep track of their engines. It is a little expensive for us commoners but might be useful in some cases. The oil is sampled and sent to a lab. They can test it for metal content and for Ph and lubricity. More money buys more detail. Some people have used the technique to determine change interval though I doubt it is worth it for that unless you are talking a lot of oil (locomotives hold ~250 gallons in the pan). A better use would be to assess the state of an engine that is in question. High readings of iron indicate a wearing bore or cam/liter, aluminum is piston maybe bearing, lead is bearings, etc. Normally it is good to have periodic samples and spot the trend but in the case of a new find you could --- Sample the oil when you change it. Good reading = no problem. Moderate = check again. High = open it up and have a look.-- If the first reading was moderate, sample again after ~50 hours on new oil. Good reading= OK, you bought a good one but they didn't change oil often enough. Moderate reading = There is a problem and you will have to decide whether to find it now or wait for it to get severe.High = The problem is getting worse, fix it now.
The second section was authored by Ed Hackett, and is version 1.2 of his OIL FAQ as posted to Usenet.