I have always broken new engines (truck & boat) in on a decent quality mineral based oil. Usually 2 relatively short period oil changes and then switched to full synthetic.
There is actually a Service Notice form Mercury Remanufacturing about this and here is what they say to do to be safe.
The oil priming procedures for 4-cycle MerCruiser engines has been updated as of 03-01-10. The Quicksilver engine oil priming tool is no longer available (NLA) through Mercury Marine. However, the oil priming tool is available at most automotive parts stores; a part number example is provided in the attachment.
Mercury Remanufacturing has received several bearing failure warranty claim requests for remanufactured engines or longblocks. Engineering analysis indicated the engines were not primed, but dry started. This caused premature engine failure.
Please remember these points:
Remanufactured engines and longblocks are hot-tested at the factory and have oil in the crankcase.
Because of storage times, an oil priming procedure is required to prevent dry start failures.
The attached service information outlines the two accepted methods for oil priming a Remanufactured engine or longblock.
“Oil Priming” Procedures for 4-cycle MerCruiser Engines
IMPORTANT: All MerCruiser remanufactured 4-cycle engines are temporarily fit with
electrical, cooling and fuel systems to perform a series of “Hot-Test” validations for quality
control purposes. Thereafter, the engine oil drains back into the pan and loses its
lubrication value from the valve-train and cylinder walls. The “Oil Priming” procedure is
required prior to initial engine start-up to reduce the potential for premature bearing
failures.
Note: There are two methods to prime an engine prior to ignition. Either can be used.
Method A - Utilizing an Engine-Oil Priming Tool (Available at Automotive Parts Stores).
(e.g. NAPA Tool Part Number: 3446)
Method B – Utilizing a Remote-Starter-Switch (Mercury Tool: 91-52024A 1).
METHOD…A…(preferred)
1. Fill engine crankcase to proper level with recommended marine engine oil.
2. Remove the complete distributor assembly.
3. Install the oil “Priming-Tool” into distributor bore and onto the oil pump shaft.
4. Rotate the oil pump with an electric drill for approximately 2-minutes or until oil pressure
registers on an oil pressure gauge.
5. Remove the priming tool and reinstall the distributor according to instructions outlined in
a MerCruiser Service Manual.
METHOD…B…(optional)
1. Fill engine crankcase to proper level with recommended marine engine oil.
2. Remove all spark plugs.
3. Disconnect the high-energy wire from the distributor and connect it to a good ground.
4. IMPORTANT: Leave ignition key in the “OFF” position.
5. Connect one end of the “Remote-Starter-Switch” to the 12-Volt red positive battery cable
terminal and the other end to the smaller (yellow-red) terminal on the starter motor.
Note: If you opt to use the ignition switch instead of the remote-starter, disconnect the
PUR wire from the coil before turning the key. Cover the PUR wire with tape to avoid a
possible ground condition.
6. Crank the engine over for approximately 15-18 seconds. Let the starter motor cool down
for 1-minute before cycling again. Repeat this process 6 more times.
7. Remove the Remote-Starter-Switch or reconnect the PUR wire to the ignition coil.
8. Install the spark plugs and plug wires.
9. Start the engine with a good supply of cool water.
03.01.10