Craig,
It is no big deal to stick new detergent oil in your rig. The only concerns would be that this new oil would loosen up any old gunk and tend to plug small orifices and/or a oil filter. I personally don't think the detergent additives in the modern oil are that aggressive.
The oil has components in it that encapsulate any contaminant and hold it in suspension so it can be flushed when you drain your oil. That is why oil turns black...it's all the junk the oil is holding within itself, junk that isn't deposited on the interior of the engine.
All this BS aside, I would put your new oil in along with a new filter and run it for a thousand miles or so. Then drain it and replace the oil and filter and follow the regular drain schedule.
Stuart
