Mercury 4-Stroke Synthetic Blend Engine Oil – 25W-40, 4L | 8M0078642

Mercury 4-Stroke Synthetic Blend Engine Oil – SAE 25W-40, 4L

The OEM-specified SAE 25W-40 synthetic blend 4-stroke engine oil for Mercury outboard, MerCruiser sterndrive, and Mercury inboard engines, sold in a 4-litre container. The synthetic blend formula delivers improved film strength, better viscosity stability across temperature extremes, and enhanced protection during cold starts and sustained high-load operation compared to conventional marine oil. NMMA FC-W and FC-W Catalyst Compatible certified. Directly supersedes earlier part number 858053KC1.

Part Details

• Part Number: 8M0078642
• OEM Reference: 92-8M0078642
• Brand: Mercury Marine
• Supersedes: 858053KC1
• Product Type: 4-Stroke Marine Engine Oil – Synthetic Blend
• Viscosity: SAE 25W-40
• Volume: 4 Litres (approx. 1.06 US gallons)
• Certification: NMMA FC-W & FC-W Catalyst Compatible
• System: Engine lubrication – crankcase

Compatible Applications

• Mercury FourStroke outboards requiring 25W-40 crankcase oil (all displacements)
• Mercury Pro XS FourStroke outboards
• Mercury Verado FourStroke outboards
• MerCruiser sterndrive engines – 3.0L, 4.3L V6, 5.0L V8, 5.7L V8, 6.2L V8, 7.4L V8, 8.2L V8
• Mercury Marine 4-stroke inboard engines
• Any 4-stroke marine engine where SAE 25W-40 synthetic blend is specified — always confirm in your engine owner’s manual

Supersession Note

Part number 8M0078642 is the current active part number and directly supersedes 858053KC1. If your parts diagram or service manual references 858053KC1, order 8M0078642.

Service Interval

Change engine oil and filter every 100 hours or annually, whichever comes first — per the Mercury and MerCruiser service manual. Always replace the oil filter at each oil change. Most Mercury 4-stroke outboards in the 75–200 HP range require 3–4 litres per change; confirm your engine’s capacity in the service manual.

Technical FAQ

• What’s the difference between this synthetic blend and the conventional 25W-40 (8M0078640)? Same viscosity grade, higher-quality base oil. The synthetic blend offers improved film strength, better cold-start protection, and greater viscosity stability under heat compared to the conventional formula.
• Is there a full-synthetic version? Yes — see the Mercury 25W-40 Full Synthetic 4-Stroke Oil – 1L (8M0078641) for the full-synthetic option.
• Can I use automotive 25W-40 instead? No. Automotive oils lack the marine-specific corrosion inhibitors and FC-W certification required for boat engines. Using automotive oil can void your warranty and accelerate internal corrosion.
• My parts diagram shows 858053KC1 — is this the right replacement? Yes. 8M0078642 directly supersedes 858053KC1.
• What if my engine specifies 10W-30? Use the viscosity in your owner’s manual. See the Mercury 10W-30 Engine Oil – 4L (8M0078638) for that grade.

Related Parts

• Mercury 4-Stroke Engine Oil – 25W-40, 4L (8M0078640) — conventional formula
• Mercury 4-Stroke Engine Oil – 25W-40, 1L (8M0078639) — 1L synthetic blend
• Mercury 4-Stroke Full Synthetic Engine Oil – 25W-40, 1L (8M0078641)
• Mercury 4-Stroke Engine Oil – 10W-30, 1L (8M0078637)
• Mercury 4-Stroke Engine Oil – 10W-30, 4L (8M0078638)
• Mercury 4-Stroke High Performance Engine Oil – 25W-50, 1 Qt (8M0078013)
• Mercury Quicksilver Oil Filter 877761K01 – 75–150 HP FourStroke
• Mercury Quicksilver Oil Filter 877769K01 – L6 Verado
• Mercury Quicksilver 100-Hour Service Kit 8M0094232
• Mercury Quicksilver 100-Hour Service Kit 8M0097854
• Browse all Mercury MerCruiser Engine Maintenance Parts

Parts Desk Insights

The 4L synthetic blend is the right choice for a complete single-engine oil change on most Mercury 4-stroke outboards and MerCruiser sterndrives that specify 25W-40 — particularly engines in the 75–200 HP range that typically require 3–4 litres per service. The synthetic blend formula is the practical step up for customers who want better protection than conventional oil without the cost of full synthetic: well-suited for saltwater use, high-hour engines, or boats that sit for extended periods between uses.

The most common customer mistake is selecting oil based on viscosity alone without confirming the correct formula for their engine — Mercury offers conventional, synthetic blend, and full synthetic in 25W-40, and the right choice depends on the engine model and Mercury’s current recommendation for that serial range. Always cross-reference with the Mercury parts diagram and owner’s manual. Always replace the oil filter at the same time as the oil — see the 877761K01 filter for most 75–150 HP FourStroke outboards.