Smitty and gs34...
Nicely done...I actually work for a Caterpillar dealer and all of what you said is correct. Oil samples will only be effective on a one time basis if there is something in the oil like gasoline, fuel, water or coolant. Oil samples are based upon trends. Even the same year and model machines can have different sample trends. I know a good way to start, and I don't know who you are planning on taking your sample to, is to bring a sample of new motor oil with you to provide the base line. This gives the basic make up of the oil (additives, etc.) that will contribute to a solid starting point. I have not had any experience with HD, but do know of many that have used this for their cages, and with a documented trend line, the OEM's can't argue the results.
Like I said, I don't know who you are taking your sample to but any Caterpillar dealer should be able to do it for about $15 per sample.
I think your best results will come from allowing the first quart of oil to drain, which will clear anything immediately around the drain plug, then capturing your sample.
If you have any questions, let me know. Maybe I can help.