To ensure they’re not needlessly dropping oil when it still has a good service life, many operators are turning to the oil diagnostic services offered by Goughs and Shell among others. In the Goughs Fluid Analysis system, pre-paid sample kits are posted to a laboratory in Christchurch and a report either posted back or made available by an internet link.

Standard tests include elemental analysis of 22 elements; tests for moisture content and fuel dilution of engine oil samples; analysis that ascertains the levels of iron, soot, oxidation, nitration and sulphur products in the oil, and viscosity tests that ascertain the viscosity at 40 degrees.
Probably even more important than its cost-saving effect is the ability of oil analysis to give an early warning of accelerated wear before it becomes a failure. In this respect, it is very useful for engines working in various applications – stationary, off-road, marine, mining, construction equipment and so on – where the environments are so different it would be difficult to predict a safe, standard drain interval. By constantly monitoring the condition of the oil, it can be changed before it degrades too far.  
Diesel oil filtration is coming increasingly under pressure as well. Unlike the bad old days when lubrication contaminants tended to drop out on the base of the engine sump as sludge, modern oils are formulated to keep them in suspension so they can be carried through to the filter. All well and good, but it means filters cop a heavier workload as well. 
In addition, EGR engines create a higher level of what is known as ‘wet soot’, carbon particles in the two-micron size range that are too small to be trapped by a normal oil filter and stay in suspension in the oil. To handle this, the industry has developed zinc-coated composite materials capable of capturing these extremely fine particles and, via the zinc, neutralising the greater acid build-up in the oil that is also a characteristic of EGR engines.
Increasingly, diesel engine makers are designing bypass filters into their systems, elements that are capable of trapping the finer particles (down to 5 microns and below) but are too restrictive of flow rate and so handle only a proportion of the oil each circuit, with the full-flow element trapping particles in the 25 micron and above range.
For light diesel engines, a spin-on combination filter with both a full-flow and bypass cartridge inside is common, explains Neil McCorkingdale from Agip Oil wholesaler TransDiesel. “Light trucks typically use two spin-on filters, full-flow and bypass, while heavy engine makers are beginning to use more cartridge systems.
“We are also seeing designs using a single filter than has part of the mesh set up for the smaller sizes to back up the 25 micron mesh of the main material. However, these are quite expensive to produce.”
While the virtually DIY spin-off approach became popular during the 1970s, he explains, there is a current trend (driven mainly by European makers) towards cartridge-style filter layouts. 
Mike Hurley from Diesel Services sees a trend towards a greater proportion of eco-friendly filter elements, their metal components replaced by biodegradable plastic and able easily to be crushed and placed in landfills.