think most would agree that modern lubricants such as Mobil 1 & Rotella 5/40 diesel synthetics (plus more) offer far greater protection than previous lubricants. The OP asks how often for fluid/filter change... i feel that most would also agree that for occasional/ consumer applications like most of us here, the change interval would be more often than say commercial/industrial applications given fluid capacity & applications (more frequent cold starts & smaller capacities give way to more contaminants)....there are those in this forum w/far greater field experience & empirical knowledge than me ( ie:g fowler, j thomas, triple r, etc) so i can only give my view....if you are really interested in how often you should change say engine & hyd fluids, just use it to what you think is an appropriate hr level (or according to owners manual hrs, etc), then get a sample analysis. the result would be a far more accurate benchmark than opinions, however experienced, would be able to offer. use your machinery in your own cond. & applications, then get it tested. go from there....:thumbsup: