I have two five-horsepower, electric, two-stage reciprocating piston air compressors. They operate up to 150 psi, at about 15% duty cycle. The environment is from 30'F to 105'F with a median temperature of about 80'F.

My questions are: Is there a better oil choice than the 30W Non-detergent specified by the maker? Could you explain [in layman’s terms] why? Should synthetic be considered? How could the change interval best be determined, the manufacturer does not give a recommendation?

I have used an ISO 68 synthetic such as Mobil SHC 68 in an application like this before with good results. Ask you lubricant supplier for a recomendation. If you do not plan on doing oil analysis you should do frequent oil changes at least yearly, more if it runs constantly. If you do plan on doing oil analysis then the sample results will let you know when to change.

I would look to a specific-type compressor oil. These types will provide lubrication with reduced deposit forming tendencies in the heads/valves of the compressor.

Based on the "30W", you should be looking at an ISO VG 100 oil.

I am more familiar with the Mobil product line. However, I am not advocating that you "only" look at their products.

We are using Mobil Rarus 427 in our two-stage recip piston air compressors with good results.

Another choice might be Mobil Rarus 800 Series (severe duty). It is a synthetic diester lubricant that is used in recip compressors (Mobil Rarus 827 is an ISO VG 100) that have a broad temperature application range of -20F to +400F. You have to be careful with this product regarding elastomer, gasket, and o-ring, and potentially downstream material compatibilities. Check the product sheet for more information.

The Rarus 427 is a mineral based product where elastomers, etc compatibilities are with regard to the mineral oil.

I'm not sure of the lower temperature capabilities of the Rarus 427 you stated (30F). Check with a rep on that.

The biggest thing that I have seen in oil analysis histories for our eight units that use the Rarus 427 is moisture. Some of this was attributed to problems with the coalescer with the automatic water trap/dump. These units will get "dirty" before any appreciable rise in acid number that would indicate oxidation stability problems, which requires dilligent thought on change frequencies.

I suspect that your oil volume is somewhat small, typical of small recip compressors. We change oil on a quarterly basis for our units, since the components are considered "critical to operation". Based on testing, we could go semi-annual. However, the current preventive maintenance schedule is working well and we need the extra margin.

Check with the compressor OEM for additional information on the lube of choice. However, I have found with our units (Ingersoll Rand), they recommend "their oil".

The last consideration is price. The synthetics are typically 4x the price of the mineral oil products. Generally, the best case for synthetics are for extreme low and high temperature requirements or special applications. It doesn't appear that your application fits the "extremes" description.

Lastly, I would be careful of the SHC for your compressor. BuckHnter is not incorrect for choosing to use the product. My concern would be deposit forming tendencies of the additive package where compressor oils are formulated with that in mind.

Good luck.

Thank you, I truly appreciate the information and the in depth reply.

Thanks for the recommendation, I appreciate your reply.