I remember testing some micro filtration bypass oil filters on our commercials in the 1980's. Basically, you fitted a T piece at the oil pressure switch and some of the pressurised oil ran through the micro filter and back into the sump via a tube fitting. The oil never then seemed to get dirty (dipstick oil was as clean as when you put it in). We tried fitting one on an old Marina van (without changing the oil) and the old oil became almost clear after a one journey of around 200 miles! The micro filters were changed regularly but the oil could be left for longer intervals. The old oils were tested after huge mileages and were still well within spec. The only downside was one of the tubes leaked, so the system was scrapped (on the recommendation of a set in his ways transport manager)!
I don't know if they are still available.
Here's one similar.