I agree with your general sentiment, but I prefer to avoid spurious maintainance items with minimal returns.

What I meant by the engine being designed for the coolant is that the type of materials used in parts that contact the coolant (so not only the engine block and head coolant ports, but also the radiator, thermostat, water pump and any piping that contacts the coolant) are selected to be compatible with the particular blent of anti-corrosion additives in the manufacturer's coolant.

So, if you take the new long life coolant and put it in an older 4-runner, which doesn't have the compatible metal parts in the cooling system, you won't get the "long life" because the internals of the cooling system are probably not compatible. In fact, you may get a reduced life and substandard corrosion protection. The blend of each of the manufacturer's coolant is different and formulated to work with the differing components in their cooling systems so it would also not be a good idea to put, for instance, GM's dexcool into a new 4runner because there would be no guarantee that it would even prevent corrosion.

The cooling system is sealed and pressurized so contamination should not be a problem. You may get a little oil in the system over time, but this will be extremely rare with a newer vehicle and you can easily check for this by taking the radiator cap off and looking at the fluid. It will not cause corrosion in a cooling system. Their prescence would indicate that you have an engine problem (probably a head gasket) that should be looked at.

I'm not current on the chemistry involved so that is the best explaination I have. There are some good descriptions on the web that I have seen that detail the exact chemicals and metals used in the various manufacturer's systems. This issue is pretty similar to the E85 enabled vehicles having special fuel delivery systems to enable the use of a larger amount of ethanol without having corrosion problems.

I totally agree that changing the extended coolant at 30K will not cause a problem. I think that the benefits are dubious at best, other than getting a good feeling. An owner's time and dollars would likely be better spent on other items.

I plan to let the coolant in my runner go until I either need to change the thermostat or I get close to the 5 year mark. Probably around 4-4.5 years. I don't think I'll be anywhere near the 100,000 mile mark by then. I should be around 60-70,000.