Short volume oil change (SVOC) usually relates to circulating lubrication systems. When the oil is changed by draining and refilling it in the system tank or stump, an SVOC occurs. But, either full or some portion of the oil remaining in the pumps, bearings, oil lines, hoses, filters, pumps, valves, heat exchangers, actuators etc. is not changed. Hence it is called “short volume oil change” as it is a partial oil change.
When the oil is drained from machines, a small quantity of oil practically always stays inside, occluded in the interior surfaces. It also gets trapped in quiescent zones, low regions, and other such nooks and crannies. Diesel engines for example, typically retain about 15 % of their previous oil, after an oil drain. So when you refill the engine with new oil, you end up with a blend of new and used oil with the ratio of 85% to 15%. This practice can greatly affect the health of your system depending on the volume and quality of the used oil. If the blend contains a small amount of used oil that is relatively healthy, the life expectancy and performance of the oil can be acceptable. But if the used oil was heavily degraded and/or contaminated, the reliability of your new oil can be strongly affected. For example, if you mix oxidized oil as little as five percent to your new oil, the oxidization stability of the new oil will decrease by more than 90%.
It is better to perform an SVOC due to several good reasons. Firstly, it prevents the system from major disruption by avoiding “dry start” conditions. When the machine is restarted, there is a risk of dry start due to delayed lubrication. It happens when the machine is lubricated but the oil has not reached some lines and components. Large systems may have auxiliary pumps that pre-lubricate the system to avoid this problem, but other machines may not. A full system drain can disturb deposits, sediment and sludge in the process. These are solid impurities that can mobilize inside the system and cause new problems unless they are removed. (E.g. – Restricted air ways, motion Impediment etc).
A full volume oil change has much more oil, time and cost associated with it, as compared to an SVOC. Hence, the service life of an FVOC should be substantially longer. When you compare a 40% SVOC (Mix 40% new oil to 60% used oil) to an SVOC, then the life of an SVOC should be 3 times, provided that the used oil is contaminant free and relatively healthy.
The Remaining useful life of a lubricant can be defined by identifying the remaining useful life of the additive system. Most additives give up their life to save the equipment and the oil; they are sacrificial. If they are not depleting, they are not working. If they are gradually dying, they are effective. They would continue to deplete until there are no reserves left, but you should obviously change the lubricant long before this point. But you can track the Remaining useful life (RUL) of your lubricant using this method. If the antioxidant concentration of new oil is 100% and you perform an SVOC when the antioxidant level drops to 25%; a 40% SVOC will result in a blend with antioxidant concentration of 55%. This means you have a 55% RUL after performing a 40% SVOC which is more than half of the designated service life of the new oil.
Performing as SVOC comes with its own risks, the most dangerous hazard being the current oxidation of the oil being drained. The oxides spread inside the oil like food colouring in a glass of water. Typically consisting of hydroperoxides and free radicals, they are highly reactive and they quickly burn through the antioxidants coming in with the new oil.
These reactive chemicals are like an infection. They spread like a contagious disease spread inside a closed room. Check for signs of oxidization in your lubricant – elevated acid number, a viscosity increase, pungent odour, dark opaque colour etc. If you see any of these signs, you need to perform a complete system flush to purge the infection. Ensure to carry out oil analysis programs to schedule oil changes well in advance, so that you can prevent against base oil oxidation. Another problem similar to oxidation is microbial contamination, which is contamination from bacteria or fungi. It is not as common as oxidation but it is just as dangerous.
A big advantage of a full volume oil change (FVOC) is that it gives you the opportunity to eliminate ghost riders from your oil. Ghost riders are hard microscopic particles that accumulate in your oil over time. They pass through the filter as they are smaller than the pore size of the oil filter onboard. For example, if you are using 10-micron oil filters, all the particles smaller than 10 microns will enter and circulate freely inside the system. The ghost riders continually grow in number and contaminate the system until the oil is changed.