Talking Torque: Oils Ain't Oils

The lifeblood of any four-stroke engine

Do you have enough coffee to stay awake during this tech piece? Or perhaps you suffer from insomnia? If so then read on.

Oils not only lubricate a four-stroke engine but contribute up to 40 per cent of its cooling capacity, so it's essential to use the right oil for your engine.

This all changed in 1953 when oil specialist Motul introduced multi-grade oils, where oil viscosity increases as the engine warms, that could be used year 'round. Most outboard manufacturers now recommend using multi-grade oils.

For example, a common outboard oil is SAE (Society of Automotive Engineers) 10W-30. The ‘W’ stands for winter, so on cold start the oil is a 10-weight (too thin to lubricate an engine when warm) but as the engine temperature rises, viscosity index improvers raise the viscosity to 30 to provide adequate lubrication in temperate climates. The viscosity index improvers are made up of pour point depressants, which are molecules that are small when cold, enabling the oil to flow freely, but expand as the engine warms to provide high temperature protection.

SAE 10W-30 oils are designed to provide a compromise between cold weather starting and warm weather protection and are usually rated from minus 20 to plus 30 degrees, although they start to lose lubricating effectiveness above 27 degrees. This may be a concern for owners of Honda's air cooled BF2A and BF2D/2.3 engines that are designed to run on the same SAE 10W-30 oil recommended for its lawnmowers and portable generators.

Specialist marine oils such as Mercury Marine's Quicksilver Performance 4-Stroke Outboard Oil are designated FC-W or Four-Cycle Water Cooled, meaning they're designed to handle the higher loads outboards are subjected to compared to automotive engines. The FC-W designation is approved by the National Marine Manufacturers (NMMA) specifically for marine engine usage and means the oil maintains viscosity under high loads better than automotive SAE 10W-30 engine oils. 

For example, automotive engines are under load accelerating or climbing hills but come off load on level ground or going downhill. Marine engines are under constant load, particularly at higher rpm. Sure four-stroke outboards run cooler than automotive engines (around 65 degrees compared to 90) to prevent rapid accumulation of salt (actually aluminium chloride) crystals in the cooling water passages, but they are under much higher loads overall. 

In four-stroke outboards, the oil helps dissipate heat from engine loading, not higher running temperatures. FC-W oils have anti-wear additives and also provide better corrosion protection in a marine environment where, during low speed operation, condensation from cold sea water around the oil sump can increase accumulation in the sump and dilute the oil. But as automotive engines run at higher temperatures condensation is way less and generally corrosion inhibitors aren’t needed. Therefore automotive oils should never be used in marine engines. 

Quicksilver SAE 10W-30 oil is also recommended for four-stroke Aqualine and Tohatsu outboards. 

Suzuki Marine sources its four-stroke outboard oils from Motul and its SAE 10W-40 oil is rated from minus 20 to over 40 degrees, making the oil better suited for tropical climates.

For these conditions Mercury Marine offers a range of SAE 25W-40 and SAE 25W-50 oils, rated from minus five to plus 40 degrees and above. The SAE 25W-40 oils are intended for naturally aspirated four-stroke Mercury outboards, while the SAE 25W-50 oil is formulated for supercharged Mercury Verado engines. Having a narrower viscosity range than SAE 10W-40 oils there's less chance of the viscosity improvers breaking down under high marine loads. If an engine will be operated in tropical climates it makes sense not to worry about lubrication below freezing!

The benchmark SAE rating for diesel outboards is mineral based 15W-40 oil marked specifically for diesels as standard 15W-40 automotive oils don't provide sufficient lubrication to handle the high combustion chamber pressures that place more load on piston compression rings than petrol engines, but I suggest checking the owner's manual for recommended SAE ratings.

MINERAL BASED OILS

These are refined from crude oil and have been the cornerstone of engine oils since internal combustion engines were introduced in the late 1800s. 

Even nowadays, some engine manufacturers recommend using mineral based oils in their outboards. Honda's SAE 10W-30, Yamaha's Yamalube 4M SAE 10W-30 and Mercury's Quicksilver SAE 10W-30 are all FC-W mineral based oils. The Quicksilver SAE 10W-30 oil I used in long term tests on Aqualine, Mercury, Suzuki and Tohatsu outboards since 2003 remained clean between changes.

Other mineral based oils include Quicksilver SAE 25W-40 oil for naturally aspirated four-stroke outboards and Yamalube 4M SAE 20W-40 oil for all but performance engines. Mercury currently states its SAE 25W-40 oil is the recommended oil for all its four-stroke outboards whereas its SAE 10W-30 oil is only for certain temperatures (temperate only) and manual start outboards because it reduces oil friction and the effort needed to hand start an outboard. Mercury's four-stroke outboard oil change kit for its four-stroke 15 to 115 HP range includes SAE 25W-40 oil as standard, not SAE 10W-30.

SYNTHETIC OILS

These are refined from natural gas and have fewer impurities than mineral based oils.

Synthetic or synthetic ester oils as they were originally known were developed in the 60s to handle the higher operating temperatures of gas turbine engines compared to piston engines. The exhaust temperature of a gas turbine can reach 1200 degrees compared to 700 for a petrol piston engine, and this reflects the internal temperatures which, of course, are higher. Mineral based oils were incapable of lubricating effectively at these temperatures, so completely man-made oils were created to cope.

Synthetic oils can also extend oil change intervals, but the main advantage is the oil retains its original SAE viscosity rating instead of the viscosity index improvers starting to break down as engine hours accumulate. 

Tests I've conducted in automotive engines show semi-synthetics remain cleaner between oil change intervals, which is why I've now used them for several years despite the higher cost. They also provide better lubrication in higher ambient temperatures.

Semi-synthetic FC-W oils include Suzuki Marine's Motul-sourced SAE 10W-40 oil and Quicksilver SAE 25W-40 designed for all outboards in tropical conditions and SAE 25W-50 designed specifically for Verado engines. 

However while semi-synthetic or synthetic fortified oils, which can have up to 30 per cent mineral-based content, are fine for everyday boating, fully synthetic oils should only be used in performance engines. If an engine is subjected to extensive trolling or low speed operation fully synthetic oils can ‘glaze’ the cylinder walls, leading to excessive blow by of combustion chamber gases past the piston rings to the sump. The compression rings don't seal as well leading to power loss under load, such as when coming onto the plane. The only solution is to re-hone the cylinder bores and fit new piston rings. 

Yamaha offers a fully synthetic Yamalube 4M SAE 5W-30 oil designed specifically for its quad-cam 4.2 litre V-Max SHO engines.

In mineral based form SAE 5W-30 was rated to lower peak temperatures (minus 35 degrees to plus 10) than SAE 10W-30 oils. But as a fully synthetic oil it actually provides better engine protection because having a 5W base it lubricates the camshafts and crankshaft faster on cold start up yet being synthetic can handle higher peak temperatures than mineral based SAE 10W-30. Fully synthetic SAE 5W-30 oils can provide adequate lubrication from minus 35 degrees to plus 35 degrees. 

Mercury Marine also offers a fully synthetic SAE 5W-30 oil but only for its VW-based diesel stern drives, not four-stroke outboards.

THE RIGHT API RATING

The API or the American Petroleum Institute stipulates minimum oil grades for four-stroke petrol engines, with the current grade being SN. However many lower output four-stroke outboards require only an SE grade of oil, which first appeared for automotive usage in the 70s. Diesel outboards need oil that complies with API ‘C’ ratings, such as CD (late eighties) and upwards.

If in doubt about the right API grade for your four-stroke outboard, I recommend studying your owner's manual which will list the grades needed.

THE WRAP

Four-stroke oils sure have come a long way since the early mono grade mineral-based oils. Choose the manufacturer's recommended FC-W oil and API requirements and your outboard should return years of reliable boating. Just make sure you follow the manufacturer's prescribed oil and filter change intervals as listed in the owner's manual, never what a dealer tells you.

Doing your own oil and filter changes will bring you closer to your investment and help you appreciate the engineering that has gone into it. Hope all those cups of coffee helped.