My opinion is that biodegradation is a method with great potencial for that purpose. Some work has been done using a Pseudomonas aeruginosa strain for that, or even waste residueslike brewery spent grain (BSG), banana skin (BS), and spent mushroom compost (SMC), to enhance biodegradation!

Bioremediation is indeed the only friendly way to dispose oil wastes. There are many publications on this subject, for a better search you can add as key words some bacteria involved in this proccess like Achromobacter, Acinetobacter,Actinomyces,,,Bacillus, Brevibacterium,Corynebacterium, Flavobacterium,Micrococcus,,,Pseudomonas, Spirillum. There are some different methods to stimulate their activity: bioventing, bioaugmentation and so on. All depend of the result you expect and quantity of oil. 

I have understood, that you have asked what is the current status and not what could be? - I have managed for many years the central waste treatment facilities of an oil and gas group in Middle East. One problem is that waste oils can be generated from various sources. The most common way of treatment is to use it as recovered oil after pretreatment (if required) as an auxiliary fuel for the operation of thermal facilities such as incineration and thermal desorption (dual fuel burner). That solves also the problem with possible metal contamination, which will be scrubbed out in the off-gas treatment, accordingly.

With my respect to all answers regarding biodegradation... biodegradation is a slow process and needs very large facilities and long time.. I think the first step is to regenerate the used oil using sulfuric acid then bleaching clay treatment to adsorb impurities from oxidation of the oil (black matter) and then addition of improvers to make the oil ready for use again, I have used this procedure and you can get 70% of the oil back ( regenerated) and some of it turns to fuel to operate the system used , then the clay which retains about 35-40% oil .. can be also recycled to remove the oil again, and regenerate the clay with 100% efficiency of the first time . maybe some few % be remain here and their ... that time you can use biodgradation or other methodes, by this method you can earn good money also.

Baktyar, I like your answer. Research with a practical target is very rare in our days.

Mumtaz! Shokran yazeelen!

Hazardous and toxic substances inhibit the biodegradation process.

these oils reused as a fuel in high temp. cement furnaces

Have you all looked at the questions? Environmentaly-frindly methods. If you use chemicals or excavate the soil or any other aggressive methods to the soil is not longer considered environmentaly-friendly. In my humble opinion. 

 You may wish to access the United Nations literature on this subject. For example see United Nations Environment Programme,“Compendium of Recycling and Destruction Technologies for Waste Oils.” UNEP Division of Technology, Industry and Economics, InternationalEnvironmental Technology Centre, Osaka, Japan

(2012)170 pp. Accessible through the UNEP IETC

website. http://www.unep.org/itec.

1. Recycling/Refining 

2. Incineration of the remaining waste in properly constructed government approved incinerators

3. It could be mixed with asphalt or bitumen for road maintenance

4. Use degreasers to clean the floor often

A major concern with recycling used motor oil is to remove the accumulated chemicals of environmental concern such as polycyclic aromatic hydrocarbons that slip past the piston rings in combustion engines and the metals that accumulate from wear of the engines. Refining and recycling followed by proper disposal of the resulting toxic wastes - as noted by Paul A L Anawe above - is a viable approach. 

The method of choice is regeneration of the spent lube oil. I have carried out this effort and the result gave a 60-70 per cent recovery. 

The heavy material residue  can be used to harden clays.

There are some good answers posted here.  First, if the focus is on bulk used oil collection and management efforts, the issue of remediation and oil-contaminated soils poses unique challenges and represents a separate problem.  Second, with respect to used oil reclamation efforts, blending of used oils to produce a burner fuel is probably the least-cost, least technologically-intensive method for recovering their energy value - if the stream(s) in question are typical of automotive/internal combustion engine oils, those streams tend to be fairly uniform in terms of contaminants present (PAH, metals, etc.), water content, and sediment, assuming no contamination or adulteration post-collection.  All that is required to produce a reasonably good No. 4 fuel is tankage for storage and a heat source to promote any phase separation and encourage gravity settling of solids.  The more contaminated the used oil streams are, and the more specialized the end-use (e.g. re-refining to produce a lubricating base oil), the greater the effort required to convert used oil to a viable end product.  Any oily wastewater, and particularly stabilized oil-water emulsions, will be especially difficult to treat, and very little oil may be reclaimed - care should be taken to segregate such streams early in the reclamation process.  Your question also refers to industrial oils - again, depending on the types of industrial oils in question (e.g. thread rolling oils, machining oils, cold-heading oils, quench oils, etc.) the compositions may be uniform and the used oil generated may actually be of very high quality in terms of suitability for energy recovery.  However, water-soluble oils (e.g. Trim-sol coolant) and certain metalworking fluids would not be good candidates for reclamation.