RE: Oil spatter
Black oil on the bottom of the airplane can by a symptom of a problem or no problem at all.
For example, if you have an engine that you have been running for sometime and the oil residue, regardless of how much or how little, starts to turn black when it was relatively clear before, you may be developing a problem. That problem might be a worn bearing inside the engine, a piston that has become scored or has a stuck ring, a loose muffler, etc. It can also be coming from a muffler, pipe or can that has become full of carbon and that carbon is starting to release into the exhaust stream. The point here is you just have to look for changes over time and correct them if necessary. Not something to worry about, just something to take care of as a part of normal maintenance.
Another example might be an airplane / exhaust / oil combination that just puts out dark oil or an exhaust stream that comes out at low velocity producing a concentrated residue. Some oils just produce darker residue than others. Some exhaust systems tend to run hot and cook the exhaust residue and produce a darker residue. Probably not a problem at all. You just have to decide what is normal for your combination of parts and pieces and act to correct a problem is you see a change.
Yet another example might be the new engine that seems to produce excessive exhaust residue .... and it never clears up. In this case you might want to start inspecting things to see if there is a problem and again, correct accordingly.
Of course in all cases, one has to assure the engine is tuned properly, is properly cooled, make sure everything is tight ans secure, etc.
There, I never once mentioned a brand name, an oil ratio, etc.