Is there a site that you can plug in different oils to see if they are compatible, or is that considered proprietary info.

Just to clarify terminology, I usually associate the term "compatibility" with whether or not two lubricants can be comingled. We talk a lot about grease compatibility.

I usually associate the term "interchangeability" with whether or not I can drain the oil and substitute one oil for another.

Do you mean compatible or interchangeable?

Thanks e-pete,
I was actually talking about interchangeability. Guess I had one of those "brain hic-cup" moments.

I don't know of a web site, but Noria will usually answer these types of questions.. and Mobil and Chevron both have 800 numbers where you can call and talk to a real engineer about compatibility. Their main purpose is to tell you what Chevron Product can replace a what Mobil product or vice versa, but they also answer questions about compatability and interchangability.
In general, almost all oils are "interchangable" i.e. can be drained, then another added. The small amount of oil film left behind is rarely a problem...and if in doubt, can be addressed by a "flush" with the new oil, before final add. The only times I can think of when interchanging is a prolem, are when changing from a ester based, to a glycol, or PAO to ester, in other words, completely different formulations. Compatibility (mixing) on the other hand is a big deal.
Almost all R&O type oils from any manufacturer can be mixed i.e. Chevron AIO and GST oils are pretty much the same as Mobil DTE ligt, medium, and heavy series.
Most AW type oils can also be mixed. What counts is that the viscosity is the same and the additive packages are at least similiar. You should never mix a Turbine oil (R&O) with an AW (extreme pressure) oil.
Gear oils are hard to determine compatibility because the additive packages can get pretty complex.
Multi-grade motor oil brands should also not be mixed.. almost every vendor has their own special additive blend, and mixing them can cause strange reactions, but it is perfectly ok to drain and refill with another type.
When in doubt, I call Mobil, Chevron, Royal Purple, etc... and ask, they are normally happy to help.