Shell’s CL-4 and FB-4 Diesel Oils: What the Testing Shows

Shell’s case for its next generation of Rotella diesel oils rests on a practical challenge: protect hard-working engines while reducing the oil components that can affect emissions equipment. Its PC-12 development work produced API CL-4 and FB-4, the categories set to replace CK-4 and FA-4. Shell’s presentation combined laboratory results, extended tests and fleet experience to explain how it approached that balance.

For buyers, the distinction between the categories matters. Shell describes CL-4 as backward-compatible with CK-4 applications when the oil meets the engine’s requirements. FB-4 is aimed at engines approved for lower-viscosity formulations. Neither category makes viscosity a matter of personal preference: Shell directs customers to the oil grade and specification their engine manufacturer recommends.

Formulation work aimed at tougher operating conditions

PC-12 is the industry development program behind both categories. Shell said its work on this generation of Rotella formulations lasted roughly three years, following earlier ingredient development by component suppliers. The process involved choosing base oils and additives, screening candidate blends, testing them, and reformulating when results fell short. Candidates then advanced to controlled engine tests and customer fleets, with some development taking place while final test requirements were still being defined.

Engine makers face a combination of demands: engines must work at higher temperatures and pressures, protect costly components and keep increasingly complex exhaust aftertreatment systems functioning. Oil has to meet those needs while using less of certain chemicals that can affect emissions equipment. Shell project leader and formulator Patrick Joyce described improved resistance to oil breakdown as one of the changes sought. Technology manager Matt Urbanak outlined the company’s approach: standard engine tests, tests run beyond their normal duration and field trials.

T4 results cover deposits, oxidation and wear

For Rotella T4 15W-40, Shell reported two piston-cleanliness results with different baselines. In the Caterpillar C13 deposit test, the merit rating was 39% above the CL-4 minimum. In the Volvo D13K4 test, the new formulation’s piston deposit rating was 58% better than the current CK-4 Rotella T4 15W-40. The first figure compares against a specification threshold; the second compares new and previous Rotella formulations. Both describe deposit ratings, not a predicted increase in engine life.

Shell also reported Volvo T13 oxidation measurements more than 91% below the CL-4 limit, and viscosity increase more than 89% below its limit. Lower readings are better for both measures. In a separate result, rocker-arm wear was 26% below a proposed Ford specification limit. Shell noted that the Ford limit was still a draft at the time of the presentation. A slide footnote says the test used 10W-30 and was applied to 15W-40, so the result is supporting evidence rather than a finalized approval for every application.

Extended testing put T5 and T6 under added strain

Shell used Rotella T5 10W-30 to show how a formulation performed when a test ran longer than its standard duration. The Volvo T13 oxidation test normally runs for 360 hours; Shell asked the laboratory to continue to 432 hours, a 20% extension. The reported oxidation result remained below the CL-4 limit. The slide says the test was run on 15W-40 and applied to the 10W-30 formulation. This demonstrates performance under prolonged test conditions, but it does not support extending a vehicle’s oil-change interval by 20%.

A separate Detroit Diesel liner-scuffing test challenges contact between piston rings and cylinder liners. Shell reported that T5 10W-30 ran for more than twice the 31-hour CL-4 minimum. That points to additional margin in a severe laboratory test; test-stand hours can’t be translated directly into road mileage or engine life.

Rotella T6 10W-30 faced a 600-hour Daimler OM471 evaluation. Two turbochargers each ran for 300 hours before deposit ratings. Shell said turbo cleanliness cleared the required minimum, while describing the test as demanding. Oxidation readings were 67% below the Detroit Diesel limit at 300 hours and 31% below the Daimler limit at 600 hours. Those comparisons are against separate specification limits, not earlier T6 results.

In the Cummins ISM wear test, which reached about 8% soot, Shell reported top-ring wear 64% below the CL-4 maximum. The company’s lab demonstration paired measurements with physical inspection, including scans of piston surfaces before and after cleaning. That gives useful context for how deposit ratings are assessed, though the figures remain test results rather than predictions for a particular truck.

Fleet trials add experience from commercial use

By early October, Shell said it had documented approximately 15 million miles of PC-12 field trials. The work covered long-haul freight, heavily loaded coal trucks, quarry and agricultural equipment, as well as Ford F-250 and F-550 trucks used in landscaping. Shell said it supplied coded test oils without identifying the formulations to customers and collected used-oil samples at service intervals. The presentation gave the clearest teardown detail for two commercial trucks, rather than the pickup and chassis-cab applications.

The featured trucks were 2023 Freightliners with Detroit DD15 engines, used on coast-to-coast routes with team drivers. Their oil-change intervals were approximately 45,000 to 50,000 miles; those are fleet figures, not recommendations for diesel pickups. Unit 756 had about 775,000 total miles at inspection. It switched from CK-4 T4 15W-40 to CL-4 T4 15W-40 at about 372,000 miles, then ran roughly 403,000 miles on the new formulation. Unit 763 had about 735,000 total miles and moved from CK-4 T4 15W-40 to CL-4 T5 10W-30 at about 315,000 miles, accumulating roughly 420,000 miles on the new oil. Both trucks had used factory-fill oil for their first approximately 50,000 miles.

Those histories put the teardown results in context: neither truck covered its entire life on PC-12 oil. Shell reported clean parts and minimal visible bearing wear after the trials, then brought components back for further assessment. The results provide real-world evidence from working commercial engines, but don’t establish that every engine will show the same condition.

Lower phosphorus is paired with added antiwear chemistry

Shell described a reduction from roughly 1,200 parts per million phosphorus in current formulations to a CL-4 maximum of 800 ppm. Because phosphorus-containing additives help control wear, the company said it added supplemental antiwear chemistry. Shell showed used-oil iron data from DD15 and Cummins X15 fleets, including severe-duty operations. Urbanak said readings for the old and new formulations largely overlapped, with no upward iron trend indicating increased wear. He also stressed that used-oil analysis can’t replace a physical inspection of every engine.

Lower sulfated ash, phosphorus and sulfur are intended to support emissions-system durability while maintaining engine protection. The presentation did not establish a specific change in pickup regeneration frequency. For owners, the selection rule remains straightforward: use the viscosity and specification approved for the engine, and follow the manufacturer’s manual.

Rollout and warranty limits

Shell tied the new oils to revised lubrication limited warranty limits. T4 coverage remains up to 10 years or 300,000 miles. T5 increases from 10 years or 300,000 miles to 12 years or 400,000 miles. T6 moves from 15 years or 500,000 miles to 15 years or 600,000 miles. The supplied material doesn’t give the full eligibility and enrollment terms, so owners should review those terms before relying on the coverage.

Shell says CL-4 and FB-4 products will arrive in phases beginning in early 2027, alongside the Jan. 1, 2027, first API licensing date. Availability will vary by product. The development story therefore ends where buyers will feel it: in the approved oil options for their engines, the timing of local availability and the terms attached to each product.

Related reading: An interview on DEF storage and diesel emissions and a look at oil viscosity and the Duramax.