HVAC Compressor Oil: All You Need to Know

The old saying is that “motor oil is motor oil,” meaning it’s all the same. That might once have been true, but certainly not today with standard and synthetic oil in a bunch of “weights” available.

It’s the same with compressor lubrication – compressor oil, and whether you have an R22 AC or heat pump or a unit using R410A is the key to which type your system requires.

This PickHVAC compressor oil guide provides an overview of the importance of compressor lubrication, the oils used, how they differ and why that matters. Common FAQs are addressed, like “How can I tell if my AC needs oil?”

Overview of HVAC Compressor Oil

The compressor in your HVAC system contains internal parts that move rapidly. This requires a special type of oil for lubrication for smooth operation. The oil also seals the compressor connections to prevent a leak.

There are two types of oil used in AC units: mineral oil and synthetic oil.  We will discuss the usage and properties of both mineral oil and synthetic oil later in this article. A strong word of advice…. make sure that you use the proper type of oil that matches and works with the specific type of refrigerant that the refrigeration compressor in your air conditioning unit uses.

Your HVAC technician should know which type to use – but one of the principles we hold firmly at PickHVAC is that the more you, the consumer/homeowner, knows about your HVAC system, the less likely an inexperienced tech will be to muck up your equipment. So, makes sense to ask:

• What are you going to do to service or repair my AC (furnace, heat pump, air handler, etc.)?
• What parts are you going to replace?
• What are you going to replace them with (universal vs OEM, for example)?
• What materials are you going to use (compressor oil, refrigerant, insulation, etc.)?
• Are you sure the materials are specified for this unit? Do you have the right stuff?

Types of HVAC Compressor Oil

Lubrication is necessary for your air conditioner or heat pump compressor to keep it functioning smoothly as it works to make your home cool and comfortable. It is imperative that the oil you use is compatible with the type of refrigerant that is in your HVAC system.  That is the key to the whole thing!

And that’s different from automobiles. Most can use either a standard or synthetic oil. Refrigeration compressors in HVAC equipment are not the same.

Below are the two types of compressor oil commonly used and any important information that deals with the function of oil in an AC unit:

Mineral oil – This type of oil, also known as MO or Naphthenic, is compatible with HCFC (hydrochlorofluorocarbons) refrigerants.  HFCF refrigerants are commonly referred to as Freon, even though that is a brand of refrigerant, specifically R22, which is outlawed for manufacture or import into the US because it depletes ozone if leaked or spilled.

Besides banned R22, other refrigerants safely used in compressors with mineral oil lubrication are:  R401A/B, R402A/B, R403B, R408A/B, R409A/B.

Synthetic oil – Synthetic oils are used in air conditioning compressors that use CFCs (chlorofluorocarbons) for the unit’s refrigerant.  This type of oil can be part of three different formulas, which will be listed below.  Synthetic oil is a newer product compared to mineral oil, and it keeps improving year after year. Each type of synthetic oil can work with a specific type of refrigerant.  Please note that there is some overlap between the types of oil. A small number of refrigerants can use either mineral or synthetic oil.

• Alkylbenzene (AB) – This is a type of synthetic oil that has been around longer than most. The following refrigerants can use this type of oil in the AC unit: R22, R401A/B, R402A/B, R403B, R408A/B, R409A/B.
• Polyolester (POE) – POE is used with refrigerants that are primarily CFCs.  It is the most-used type of oil in air conditioning compressors. Here is a list of refrigerant codes that can be used with Polyolester: R124a, R404A, R407A/C/F, R410A, R507A, R417A, R422A/D, R424A, R427A, R428A, R434A, R437A, and R438A.
• Poly Alkyl Glycol – One the least common types of synthetic oil, it can be used in air conditioning systems that use CO2 and R744 for refrigerants.

There are some types of refrigerant that are compatible with both mineral oil (MO) and synthetic oil.  Some of these popularly used refrigerants are R421A and R407C.  Please consult your user manual or contact an HVAC technician to determine what type of oil is best for your AC unit.

How to Tell if your Air Conditioner Needs Oil

Is your compressor low on oil? It could be quickly harmed if so.

The purpose of oil is to keep all the parts lubricated and also help cool any of the moving parts that are located in the compressor.  There is also an added function for compressor oil – an oily film protects the rubber seals in the lines and at any joints or connections.  In short, the oil seals the entire compressor.

Here are the telltale signs that your compressor may need oil – and whether the damage can be repaired (yes) or the compressor must be replaced (no):

• Your air conditioner recently started making an odd or strange humming noise (yes).
• The compressor has a hard time starting or takes longer than normal to start working (yes).
• The compressor or lines are showing signs of a leak (yes).
• Smoke or an odd, burning smell is exiting the compressor (probably no).
• The compressor on your AC unit seizes or freezes up….it stops working (definitely no).

How to Check the Oil Level in your HVAC System

Call a technician and pay them to do it – That’s probably the best way to ensure proper AC or heat pump maintenance.

However, if you enjoy the DIY process, there are a couple of ways you can check to see if you have a low oil level in your air conditioning unit’s compressor.

1). Check the oil level on the compressor’s sight glass that is common on many HVAC systems.  The sight glass will show you the oil level and you can easily tell if you need to have an HVAC technician come out to add oil and possibly more refrigerant.

2). A second way is to hire a professional HVAC technician to come out and do a thorough inspection of your air conditioner. Yes, we know we mentioned it, but we thought it bears repeating. Compressors are expensive – about $1,200 to more than $2,000 to repair according to our AC Compressor Repair Cost Guide, so maybe it makes sense to spend $100-$150 on a pro to make sure you are good to go.

Should I Add Oil to My AC Unit?

Air conditioning units shouldn’t burn oil. But they might leak. If your compressor lubrication/lubricant is low, then it means there has been a leak which has caused it to lose oil.

And yes, oil should be added, but not before the leak is repaired.

Contact a professional HVAC technician to ensure that the leak is fixed and the proper oil is added. After adding AC compressor oil, the technician should check/test to determine that the entire AC system is sealed and functioning properly.  Lack of oil or an oil leak can cause major problems to your HVAC system.

Whenever you recharge your HVAC system with refrigerant, you should also add or replace compressor oil since the old refrigerant will have some of the original oil mixed with it. In short, some will be lost. Complete replacement of the oil, rather than just topping it off, is preferred.

If you are mechanically inclined, you can add the oil yourself by following the manufacturer’s directions in your air conditioning unit’s owner’s manual.

Key Points About Compressor Oil Maintenance

Let’s recap the information here:

1). Your AC needs oil for the compressor to function properly and efficiently.

2). Mineral oil (MO) and synthetic oil are manufactured.

3). The type of oil that your AC unit requires depends on the type of refrigerant that your unit runs on.

4). Oil shouldn’t leak, so if your heat pump or AC is low on oil, the leak should be repaired before oil is added.

5). This is one HVAC repair we think is best left to an air conditioning professional who does this type of work on a regular basis. If you’ve got a bad AC capacitor or need the coil cleaned, those are good DIY projects. This one isn’t, nor is an AC recharge.