I would suggest using some sort of atomic emission spectroscopy. In fact, there is a paper on Research Gate detailing the results from using Inductively Couple Plasma-Atomic Emission Spectroscopy (ICP-AES) for just this sort of analysis.

Rauckyte-Zak, T., Hargreaves, D.J., and Pawlak, Z., "Determination of heavy metals and volatile aromatic compounds in used engine oils and sludges," Fuel, Vol. 85, No. 4, 2006, pp. 481-485.

Hope this helps,

Neal

Article Determination of Heavy Metals and Volatile Aromatic Compound...

I would suggest using some sort of atomic emission spectroscopy. In fact, there is a paper on Research Gate detailing the results from using Inductively Couple Plasma-Atomic Emission Spectroscopy (ICP-AES) for just this sort of analysis.

Rauckyte-Zak, T., Hargreaves, D.J., and Pawlak, Z., "Determination of heavy metals and volatile aromatic compounds in used engine oils and sludges," Fuel, Vol. 85, No. 4, 2006, pp. 481-485.

Hope this helps,

Neal

Article Determination of Heavy Metals and Volatile Aromatic Compound...

ICP-MS would give better results. It has very low detection limit. It depends on what you want to analyze as well. You need proper protocol for extraction from oil.

There are several papers which can help you. 

X-Ray Flourescence may be the answer. It may not offer the detection limits as good as ICP-MS but the sample handling and preparation if by far easier.

Hi!

I think its better that you begin first with XRF to find which kind of metals there and then you can decided which metals you want to study in detail with ICP or AAS.

Best regards

As Mohd Faidz Mohamad Shahimin points out, before getting to AA or ICP analyses, you need to have a method to digest the oil first. EPA Method 3031 outlines a protocol that should work in your case (http://www.epa.gov/wastes/hazard/testmethods/sw846/online/3_series.htm).

concentrated acid (HCl, HNO3) digestion with microwave (i.e. Anton Paar or MARS Express series) followed by atomic absorption or ICPMS, good luck.

The answer to this depends upon what you are really trying to measure, do, and/or how much money you want to spend.  If the goal is just to get an idea of how much "wear" metal are in your oil, the simplest approach would be to determine an easy/cheap to determine metal that tends to build up quickly. This ref . http://www.horiba.com/fileadmin/uploads/Scientific/Documents/Emission/ICPPETRO03.pdf   suggests that iron would probably be the best one to go for - its concentration varies more as a function of time-in-the-engine (wear)  than do other metals, its concentration is relatively high throughout (which means you don't have to do a "trace" analysis or prepare much sample),and there are lots of cheap/ simple ways to determine it.  For example 20-50 ul of oil soaked into a gram or so of magnesium nitrate within a crucible can be mineralized in 2-5 five minutes by heating it in with a bunsen burner or 500C muffle furnace,  the ash solubilized in nitric acid & then turned into a solution whose  iron content is easily measured by adding a solution of ammonium thiocyanate,& diluting to a convenient volume (eg 5 or 10 cc)  & then measuring its absorbance at about 540 nM with a filter photometer.  On the other hand if you are doing "compliance" work  and/or have to fill in lots of blanks in a form, then something like ICAES might be better.  If he's  got really big bucks, I'd tell my boss that we/he "need(s)" to get the latest/greatest ICPMS.

I suggest you just use the ASTM D4927, D4951 and D5185 already used in automotive industry.

You might consider USEPA SW-846 Method 3031which is an acid digestion procedure which can be combined with ICP-AES, ICP-MS, FAAS, or GFAAS.

http://www3.epa.gov/epawaste/hazard/testmethods/sw846/pdfs/3031.pdf

"Spectroscopic Analysis of Petroleum Products and Lubricants", by Kishore Nadkarni, ASTM Monograph 9, is an excellent reference covering test methods and applications. Chap. 22 is specific to analysis of used oils.

I recommend Islam's answer

Best Regards