I am going to have to get my tribology textbooks out one day soon. All this lubrication discussion. True lubrication engineers can feel free to correct me on the details below.
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Originally Posted by DSAviate
1. How is it that the dark 5w30 at 6000 miles is lubricating ever bit as well as the highly translucent stuff I put in 6 months ago?
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Well, it may be counter-intuitive, but the colour of the oil has no bearing on the lubrication quality of the oil. Clear or translucent oil can be broken down and damaged, and dirty oil can be fine. If you change the oil in your diesel engine, it will be black in a couple of minutes. The material providing the colour is small enough not to impact the lubrication quality, until the point at which the viscosity starts to change. Particles large enough to matter for wear impact are taken out by the filter. The oil has several jobs, including lubrication, cooling, etc, but one of the most important is to keep contaminants in suspension. It is probably a bad analogy, but if you dissolve a tsp of salt in a glass of water, the salty water doesn't feel gritty. Same principle. Incidentally, this also relates to why oil sampling doesn't tell you much about the oil, but instead about the compartment it came from. Metallic elements in the oil measured through typical analysis are not damaging the oil, they are simply providing history on what types of metals wore in the engine. The exception to that is TBN/TAN reporting, which does tell you about the depletion of additives and thus when to change your oil, and silicates, which tell you about either dirt or assembly lubes.
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Originally Posted by DSAviate
2. As the transmission gears, chains, etc. lose metal (and they do erode metal), where do those metal shavings, molecules, etc. go? My E53 has an oil filter for ONLY the engine oil, and I haven't yet located any appropriately sized drain plugs with magnetic tips to help corral these life robbing varmints.
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If they were losing as much metal as you suggest, the transmission would be very noisy very quickly. The small amount of metal that wears is held in suspension by the oil. If the transmission had rough edges and poor machining from the start, then a magnetic plug would help stop those pieces circulating. There aren't a lot of pieces being broken off once the gears are run in. A magnetic plug isn't a bad idea at all, but the engineers who designed the vehicle couldn't justify the pennies it would have cost, essentially because the benefit is less than that tiny cost.
You are comparing sealed compartments like transmissions and diffs with the engine, and they are each very different. The engine has a built in source of contamination, namely the combustion process. It generally has a filter because it needs one (not all engines do, it depends on the design). A design engineer can pressure lube a transmission and then add a filter to the circuit, but that isn't worth doing in an automotive application because the gears will last long enough without that level of sophistication and cost.
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Originally Posted by DSAviate
3. Higher loads equal greater wear. Do you try to get your MXX to produce rated hp often? Are you pulling close to the 6,000 lbs limit routinely? The bits of metal fragment roaming around our transmissions, transfer cases, and differentials suspended in "lifetime oil" are taking their toll until they're drained out.
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Lots of things result in greater wear, but the issue is are you crossing over beyond the safety factor that the engineer designed the system for? It isn't a linear relationship. Higher loads don't directly cause much difference in wear because the loads are well within the design limits for the lubrication, and are usually applied for a portion of the operating cycle of the equipment. Fatigue wear isn't a factor until you get to much higher power outputs, usually. Heat is a big factor, and towing is a good example of loading up a system and creating more heat. If that heat is managed by a well-maintained cooling system, towing in and of itself won't destroy fluids.
It is all an interesting discussion. Many owners wrestle with how often to change fluids, and which oil to use. My opinion, and it is only my opinion, is that the manufacturer's recommendations are pretty reasonable, and if someone wants to change them more frequently then it is a cheap way to feel good about maintenance. More frequently can mean simply doing them 20% more often. Doubling the frequency of planned changes is quite a step, and doing them more often that that seems extreme to me. We have the benefit of very high quality oils these days, and the same for filters. Any of the name brands that meet the manufacturer's spec will do fine. Your vehicle won't know the difference between different oils that meet the same spec. Not following the manufacturer's specs is like playing roulette, but sometimes people win at roulette.
As to your planned fluid changes, absolutely use new crush washers. I don't see why you wouldn't go to a 5-40 for Florida temperatures and towing. I wouldn't use LM, but that is just me. I would use Castrol or Mobil 1. For the diff, I would follow BMW recommendations on viscosity.