Although modern gasoline engines often specify a Starburst oil rather than an API SN/SM/SL/SJ-licensed oil, Heavy Duty Engine Oil (HDEO) can sometimes be used in its place at the risk of poisoning emission control components slightly quicker. Starburst oils are passenger car engine oils that meet the latest ILSAC specification (currently ILSAC GF-5, previously ILSAC GF-4). The phosphorus limit for API SM & SN is 800 ppm of phosphorus while it's 1000 ppm for API SJ & SL. The main differences between HDEOs and Starburst Oils for older flat-tappet engines are maximum phosphorus levels and friction modifiers. Starburst oils have friction modifiers for better fuel economy (1-2% potential improvement), which HDEOs generally do not have. Racing Oils (like Valvoline VR-1) have higher phosphorus levels but they also have reduced detergency compared with Starburst oils, which means they require more frequent oil changes to prevent sludge formation. In contrast, HDEOs have greater detergency than Starbust oils which means that they are much better at keeping gasoline engines clean.
As with API CJ-4, the current API CK-4 specification limits phosphorus to 0.12% (1200 ppm) maximum. More specifically, the phosphorus limit for API SN-RC (ie, Resource Conserving, for ILSAC GF-5 viscosity grades, typically SAE 0W-20, 0W-30, 5W-20, 5W-30 and 10W-30) is 0.08% but plain API SN (for non-ILSAC GF-5 viscosity grades, XXW-40) is not limited. Therefore, 30-grade oils approved for both API CK-4 and API SN have a maximum phosphorus limit of 800 ppm, which is why many HDEOs have gone from a dual rating (eg, CJ-4/SM) to a single rating (ie, CK-4). Ford has found that some CK-4 oils with phosphorus levels lower than 1000 ppm did not provide acceptable valve-train wear protection. As a result, the Ford WSS-M2C171-F1 specification includes a minimum phosphorus requirement of 1000 ppm. When checking an oil's product data sheet, look for the following statements about the specifications:
| Statement | Meaning |
| Approved | Passed the specification's tests and is approved. |
| Suitable For Use | Not necessarily tested but deemed suitable for use based on being formulated with the same additive technology as viscosity grades that have been tested and passed. Potential approval issues could involve the specification's maximum phosphorus and sulfur content requirements. |
| Meets Specifications | Not tested but meets the requirements of the specification. |
Since it can be difficult to find older API service categories in North America (such as CI-4+, CI-4, CH-4, etc), a safe option for flat tappet engines without catalytic converters is to use a dual-rated CK-4/SN heavy duty engine oil (ie, approved for API CK-4 and suitable for API SN) that is also rated for the Ford WSS-M2C171-F1 specification (ie, approved or suitable for use).
If your car has a flat-tappet camshaft and no emission controls (catalytic converter & O2 sensor), then you should be using oils that contain enough phosphorus (in the form of ZDDP) to protect the valve train. Any currently available HDEO contains more ZDDP than what is needed for OEM-style valve trains. Aggressive aftermarket valve trains would probably be safer with CH-4 or CI-4 oils having 1300-1400 ppm of phosphorus but there is more to an ideal engine oil than total ZDDP content. If your engine is not burning oil, the higher levels of phosphorus in an HDEO could help to prevent it from burning oil in the future at the risk of shorter emission system life.
The following pages list dual-rated HDEOs available in North America. If the oil you are interested in trying is not on your local store's shelf, you can order it from a distributor. Engine oils with a certification equal to or lower than API CF are NOT listed. See SAE Viscosity Grades for Engine Oils for viscosity specifications.