Oil in a typical Diesel engine will look like molasses very quickly. Sometimes after a trip around the block. Diesel engines are like that. Don't worry about it.

My guess is that the best time to send an oil sample for analysis is at the end of a typical oil usage cycle. As in right when you change. Part of the analysis is to suggest that you're waiting too long between changes or that you can leave the oil in longer without detriment.

Sixto
95 S420
87 300SDL