2023 Peterbilt 579 — Transmission Fluid & Service Guide

Quick answer

• Transmission make/model you may have: Eaton Fuller 10/13/18‑speed manual (RT/RTLO series), Eaton UltraShift Plus AMT, Eaton Endurant HD 12‑speed AMT, PACCAR TX‑12 AMT, or Allison 4000‑series automatic (e.g., 4500/4700). Verify by the transmission ID tag on the case.
• OEM fluid spec:
  • Eaton Fuller/UltraShift/Endurant: Eaton PS‑386–approved synthetic manual transmission fluid (typically SAE 75W‑85/75W‑90 or SAE 50 MTF per the approval list; not ATF).
  • PACCAR TX‑12: ZF/PACCAR‑approved synthetic MTF meeting ZF TE‑ML 20C (75W‑80/75W‑85).
  • Allison 4000: Allison TES 668 (latest) or TES 295–approved full‑synthetic ATF.
• Drain & fill capacity (approx.):
  • Eaton Fuller manual / UltraShift Plus: 14–16 qt (13–15 L)
  • Eaton Endurant HD: 15–17 qt (14–16 L)
  • PACCAR TX‑12: 15–17 qt (14–16 L)
  • Allison 4000: 19–25 qt (18–24 L), sump‑dependent
• Full overhaul/dry fill capacity (approx.):
  • Eaton Fuller / UltraShift: 16–18 qt (15–17 L)
  • Endurant / TX‑12: 17–19 qt (16–18 L)
  • Allison 4000: 38–42 qt (36–40 L)
• Filter part numbers:
  • Eaton Fuller manual: no external filter; clean magnetic drain/fill plugs and internal screen where equipped.
  • UltraShift Plus / Endurant / TX‑12: external transmission oil filter; part number varies by model/build—verify with dealer or service manual.
  • Allison 4000: internal main and lube filter kit—verify with Allison by serial number.
• Recommended service interval:
  • Eaton PS‑386 linehaul: up to 500,000 miles (5 years); severe/towing: 250,000 miles (3 years). Some Endurant/TX‑12 applications allow up to 750,000 miles—confirm by model.
  • Allison TES 668: fluid up to 300,000 miles; filters 100,000–150,000. Severe: fluid ~150,000; filters ~75,000.

Fluid type explained

Your 2023 Peterbilt 579 was offered with multiple gearboxes. Most Eaton and PACCAR automated manuals (AMTs) and Eaton Fuller traditional manuals use a dedicated manual transmission fluid, not ATF. The current Eaton factory callout is PS‑386 approved synthetic MTF. These fluids are typically SAE 75W‑85/75W‑90 or an SAE 50 manual transmission oil specifically on the PS‑386 approval list. They provide synchronizer and clutch‑actuator compatibility and are different from GL‑5 hypoid gear oils used in drive axles—do not substitute axle lube.

PACCAR’s TX‑12 (TraXon‑based) uses a ZF‑approved synthetic manual transmission fluid meeting TE‑ML 20C. Again, this is an MTF, not ATF. Using the correct approval is more important than the label weight because friction characteristics are tightly controlled for AMT durability and shift quality.

If your 579 has an Allison 4000‑series full automatic, it requires a synthetic ATF approved to Allison TES 668 (preferred) or TES 295. TES 668 offers improved oxidation stability, extended drains, and better low‑temperature performance; TES 295 remains acceptable if already in service.

Always confirm the exact fluid by transmission model and serial in the operator’s manual or OEM lube chart for your VIN.

Service procedure

Two practical approaches are used in the field:

• Drain & fill (most common)

  1. Warm the transmission to operating temp and park level. 2) Remove the fill plug first to ensure it’s free. 3) Drain completely. Clean magnets and any internal screen. 4) Replace sealing washers/O‑rings as required. 5) Refit drain plug to spec torque (refer to service manual). 6) Refill with the approved fluid until it reaches the bottom of the fill hole (or to the sight level if equipped). 7) For AMTs, perform a clutch/shift calibration if the procedure calls for it and check for leaks.
  • AMTs (UltraShift/Endurant/TX‑12): Replace the external oil filter at the same time. Prime the filter if specified by OEM. Some models require key‑on pump prime or a service tool routine; follow manual.
  • Allison 4000: Replace both internal filters during the drain & fill. Refill with TES 668/295, run through ranges, recheck hot level with dipstick per Allison procedure. Reset prognostics if equipped.

• Full exchange/overhaul fill Used when the unit is rebuilt or the case has been opened. Fill to the dry capacity, then verify level after a short run‑in, as air purges from clutch housings and passages (AMTs/Allison). Avoid power flushing through coolers; use gravity drains and multiple fills if chasing contamination.

Service interval

• Normal linehaul (light to moderate loads, mostly highway):

  • Eaton PS‑386 (Fuller/UltraShift/Endurant): Inspect level every PM; change up to 500,000 miles or 5 years (some Endurant/TX‑12 approvals allow up to 750,000—use the OEM chart for your exact model).
  • PACCAR TX‑12: Follow the PACCAR/ZF schedule for TE‑ML 20C oils—typically comparable to Eaton’s extended intervals in linehaul.
  • Allison 4000 with TES 668: fluid up to 300,000 miles; filters 100,000–150,000 miles (time limits commonly 48 months fluid / 36 months filters).

• Severe duty (frequent stops, heavy GCW, mountainous routes, extreme temps, PTO time):

  • Eaton/Endurant/TX‑12: 250,000 miles or 3 years is a conservative target; shorten if oil analysis indicates.
  • Allison 4000: fluid ~150,000 miles; filters ~75,000 miles, or use Allison prognostics.

Oil analysis is highly recommended on tractors with high utilization or mixed service; it can safely extend or shorten intervals based on actual oxidation, viscosity, and wear metals.

Common transmission problems on this generation

• Eaton UltraShift Plus / Endurant AMTs: Clutch actuator or XY‑shifter calibration issues, input/output speed sensor faults, and wiring harness chafe. Low/incorrect fluid can cause harsh or missed shifts. Software updates and proper clutch learn after service resolve many complaints.
• Eaton Fuller manuals: Hard shifts or gear clash when MTF is contaminated or wrong spec (GL‑5). Countershaft bearing noise from low oil level; watch for leaks at PTO covers and rear seal.
• PACCAR TX‑12: Intermittent mechatronic faults, clutch judder on starts if calibration is off, and oil seep at selector cover. Keeping the correct TE‑ML 20C MTF and performing teach‑in after clutch service are key.
• Allison 4000: Cooler line leaks, PTO seal seep, torque converter clutch shudder when fluid is degraded. Using TES 668 and keeping filters clean greatly reduces heat‑related issues. Always verify hot level with the proper procedure.

If you’re unsure of your exact transmission model, record the ID tag information and consult the dealer or the service manual—this is also necessary to obtain the correct filter kit part numbers.

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