You may not know it, but if your machine is a more than a few years old,


Stop! Don’t Blend Your Lube Oils Until You Have Read This

You may not know it, but if your machine is a more than a few years old, your oil is almost certainly a blend of differing oil Groups. This is because oil companies phased out supply of many Group I oils and introduced a new, improved Group II formulation that had a similar label as the old Group I product. This ad hoc blending of oils can be detrimental to your equipment if you are not aware of the problems.

Firstly, What is the Difference between Oil Groups?

As you know, mineral oil comes from the earth, a product of decay made up of carbon material that has decomposed over an eternity. Crude oil is refined into a base stock, with additives to produce a lubricant to suit your specific application. The more the oil is intensively refined, the better most of the oil properties become. Mineral oils are graded in Groups I, II and III – Group I being the least refined. Synthetic (man-made) oils are graded in Groups IV and V, and are substantially more expensive, but offer a greater service life-cycle.

Synthetic lubricants were born out of a need for better lubricating properties, and the unavailability of crude oil in Germany. Aviation was the first application for synthetic lubricants, as they are superior to mineral oils at cold start up and cold climates. Synthetic oils have a higher Viscosity Index, offering better viscosity stability at varying temperatures. Group IV synthetic oils are man-made from PAO’s (polyalphaolefins) and Group V oils are made up of polyesters, phosphate esters, di-esters, alkylated benzenes and other synthetic molecules.


Another way of comparing the various refinement of oil is to compare the oil to a fluid with balls of varying sizes in suspension. The lower the Group level of oil, the greater variety of ball size and shapes (which represent the types of molecules). Group 1 oils will have millions of differing atomic compounds (molecules) making up the oil. As the oil is refined, the oil is more uniform and the balls are the same size. Synthetic oils have only one size ball.

Group Oil millions of compounds
Group I Oil millions of compounds
Group IV & V synthetic Oil
Group IV & V synthetic Oil


Did you Know?

BioKem baby oil not available in stores.
No fossils or babies were harmed in the making of this oil.
The brown colour in oil is mostly representative of its sulphur content. As you go up the Group levels the sulphur content reduces and the oil becomes clearer, sometimes as clear as water like baby oil.

BioKem baby oil not available in stores. No fossils or babies were harmed in the making of this oil.

You may also hear Group III oil referred to as a synthetic but it is not, it just behaves more like a synthetic oil!
BioKem baby oil not available in stores.
No fossils or babies were harmed in the making of this oil.




Group I oils have been available and in service ever since crude has been refined, but as technology and refining processes have improved, Group I oils have been phased out in favour of Group II oils. Group II oils are affordable due to the scale of economy and demand, for consumers this is good, as you are getting a quality oil at a cheaper price. As a consequence, Group I oils are now not widely available except on special order.

Industrial Consequences

As Group I oil is now unavailable in lighter grades, industry is often topping up Group I lube compartments with Group II oils, and hoping for compatibility. In-service Group I oil will probably have waste by-products of oxidation in solution caused by depleted antioxidants (a primary cause of varnish), as well as other contaminants such as wear metals, environmental contaminants and depleted additive packages. By adding make up Group II oils, the new antioxidants will be consumed and depleted by the Group I oil, causing varnish potential created by further waste by-products agglomerating.

So, before adding more Group II oil to your Group I oil, check with your supplier for compatibility, ensure your Group I oil is clean and free of contaminants, has enough antioxidant or remaining useful life left, and check to see if it really is necessary to mix the two! Get your lab to conduct an RPVOT Oxidation test too.

Warning – If your Group I oil is beyond recovery (oxidised), don’t try to rejuvenate it by adding Group II oil to it, drain it, flush your system and add a fresh charge of Group II oil.
Rotating pressure vessel oxidation test (RPVOT) - ASTM D2272, formerly called the rotating bomb oxidation test (RBOT) Rotating pressure vessel oxidation test (RPVOT) – ASTM D2272, formerly called the rotating bomb oxidation test (RBOT)

The Twisty Turny Bit

The tricky bit is that the blend of oils can fool laboratory results into thinking that antioxidant levels are within an acceptable range as the charge of new Group II oil skews the result. Bear this in mind and look for other signs such as visual varnishing and staining. If you are still not sure (if you can) then inspect the oil cooler elements on the off duty unit for staining, indicating that the oil has broken down and is shedding its additive pack. You may need to remove the blend of oil, treat the varnish, flush the machine and replenish with new Group II polished oil.

Tip: Make sure your new oil is polished (filtered) before use, as it will not meet equipment OEM cleanliness (ISO 4406) standards as supplied by the oil reseller!

ae7c3a50-0a6e-4107-ae9c-740c4bbee180 ISO 20/18/16 NAS 10 sample from ‘new’ oil delivered in a tote
3bdc1b3a-1cc9-4bd7-80bc-aa46abd91415 ISO 15/12/10 NAS 4 – Polished with 2¼m microglass media

When is it OK to Blend Oil Groups?

To improve mineral oil properties even further, manufacturers do blend synthetic base oils with mineral base oils, to produce what is known as not surprisingly, a blended oil. These oils are common in gear oil applications as synthetics are too light for these heavier applications.

Pros and Cons

Group I oils were cheap and readily available, but no more. They have a greater tolerance to particle and water contamination, however degraded Group I oil can change its molecular structure, and become carcinogenic.

Group II oils are readily available and cheap, can withstand higher operating temperatures, have increased service life, better wear protection and improved lubricity. They have a lower threshold for varnish causing contaminates though.

If you have clean oil and moisture and particulate is well managed on your site then a Group III oil will provide your machines with much better wear protection and an improved Viscosity Index.

Managing Your Oil Cleanliness


There are many solutions on the market today to mitigate moisture and particulate contaminant. BioKem uses a range of units and for the best results you may want to consider a vacuum dehydrator VDOPS type unit. They are effective at removal of dissolved, free and emulsified water, entrained gas, and particulate for heavy gear oils right down to very light oils of any Group. They are the Gold Standard of the filtration industry. These VDOPS units have class leading microglass filtration media to remove particles as small as 2¼m.

If you want some advice on specifying or sizing a suitable unit for your Plant, then let us know.

Summing up

If you can avoid ad hoc mixing or blending old and new oils, your lube circuits and rotational equipment will benefit. In critical close tolerance rotating equipment, it’s just not worth the consequences of blending.

If possible, Don’t blend Drain, Flush, Polish and Refill!

P.S. Some oil brands and formulations are more prone to problems when blended and your manufacturer should know and can advise on this.

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