What oil, oil capacity and filter does a MerCruiser MCM 4.3L engine require?

Most MerCruiser MCM 4.3L petrol engines use Mercury or Quicksilver 25W-40 four-stroke marine engine oil and take approximately 4.25 litres (4.5 US quarts) when the filter is changed. Mercury 35-866340K01 is a common genuine filter reference, but confirm both the filter and oil capacity against your engine serial number, particularly with a remote filter installation.

Check the engine serial number

The MCM 4.3L name covers several generations of GM-based V6 engines, including carburettor, EFI and MPI versions. The serial number identifies the correct Mercury parts listing more reliably than the boat's registration year. The engine may have been built earlier or replaced during the boat's life.

Look for the engine identification label or plate, often around the flame arrestor cover or engine block. Record the complete number. Check whether the oil filter screws directly onto the engine or sits on a remote bracket connected by oil hoses.

The usual service fill is approximately 4.25 litres including the filter. Sump arrangements, remote hoses and the amount of old oil extracted can affect the refill. Use that figure as a guide; the correctly checked dipstick determines the final level.

Choose the engine oil

Mercury or Quicksilver 25W-40 marine four-stroke oil suits most installations. Look for NMMA FC-W approval. If the engine manual requires catalyst-compatible oil, choose an FC-W Catalyst Compatible product. Mercury's marine 25W-40 synthetic-blend oil is a common service choice.

• Buy five litres: this normally covers the oil change and leaves some for topping up.
• Check an older engine's manual: some older manuals specify straight SAE grades according to ambient temperature. Follow the applicable Mercury guidance rather than choosing a thinner automotive oil by habit.
• Keep the fluids separate: the Alpha or Bravo sterndrive uses its own specified gear lubricant, separate from the engine sump.

Do not use a diesel engine's 15W-40 recommendation simply because it appears in another marine manual. The MCM 4.3L is a petrol engine. Follow its lubrication specification.

Select and fit the filter

Mercury 35-866340K01 is a widely used service reference for these GM-based MerCruiser engines. Part numbers can be superseded, so give the supplier your engine serial number and ask them to confirm the current Mercury or Quicksilver replacement. For a remote installation, check the filter head and existing filter too.