Fuels ain’t fuels #2 – Regular or Premium?

This article is reproduced from the May 2019 edition of our Cruising Helmsman Magazine.

In the second of this series on outboard maintenance, Andrew Norton discusses the issue between the different fuels available and how they interact with both the oil and the machinery.
There has long been a belief that if in doubt about fuel quality always use a higher octane or premium fuel.
Providing a semi-synthetic oil is used, all two-stroke tender outboard and inboard auxiliaries, such as Blaxland Chapman and Vire, can be run on premium 95 RON unleaded.
Using mineral based oils in conjunction with 95 RON will prevent the oil staying in suspension with the fuel, resulting in dry cylinder bores. Never ever use premium 98 RON unleaded, the aromatic additives package used to raise the octane to 98 will prevent even semisynthetic or fully synthetic oils from staying in suspension with the petrol.
Although using 95 RON does necessitate using semi-synthetic oil the benefits of doing this are reduced oil smoke emissions, less smell in following seas and cleaner spark plugs. Of course, also, less risk of buying fuel tainted with ethanol.
However, four stroke engines that have fixed ignition timing are designed to run on lower octane petrol such as 91 RON. Using premium 95 RON will cause premature ignition in the combustion chambers, damaging the piston crown/s. This means using 91 RON at all times but potentially risking buying fuel that has some ethanol in it: so the Fuel Testers kit described in part one of my fuel series should be an essential part of any onboard engine tool kit.

Marine four strokes operate under entirely different load conditions to automotive engines and live in a hostile corrosive environment. Therefore, automotive oils simply are not suitable for marine engines.
Automotive engines are under load only when the vehicle is accelerating or climbing a hill. On level roads and downhill the load falls off, giving the engine a ‘breather’.
But, once above fast idle, marine engines are under constant load. Because all the four stroke engines mentioned in this article are raw seawater cooled they operate no hotter than around 65 degrees Celsius compared to average running temperatures of 90° for automotive engines using coolants. If a raw water-cooled marine engine was to operate hotter than 65° there would be a rapid accumulation of salt crystals, which is actually aluminium chloride in alloy engines, that would clog the cooling passages.
So although raw water cooled engines run cooler, they run under constant load necessitating specific marine oils.
All of these are certified FC-W or four cycle water cooled by the National Marine Manufacturers’ Association (NMMA) of the United States to not only handle the constant load, but also provide far better corrosion protection when the engine is disused.
Society of Automotive Engineers (SAE) engine viscosity ratings are different than automotive oils to reflect the different operating conditions.

2 stroke outboards
4 stroke oils

So while a marine oil can be used in an automotive engine, the reverse should not be used in a marine engine, although some air cooled outboards can use land-based power equipment oils suitable for lawnmowers and portable generators.
The SAE rating describes the effectiveness of lubricating oil to protect an engine under a range of operating conditions. For example the ‘W’ in viscosity ratings means winter. If an oil has a 10W30 rating it means when the engine is cold it is a 10-weight, but as the engine temperature rises the oil thickens to a 30 weight to handle up to 35° of ambient temperature.
Using a 25W40 oil in temperate climates may create cold starting difficulty due to the thickness of the oil. This is why Suzuki opted for a Motul 10W40 rating to suit both temperate and tropical operating conditions.
The Motul oil is also semi-synthetic to provide more effective protection between oil and filter changes. Semisynthetic oils maintain the original SAE rating between services, whereas mineral based oils lose the ability to maintain the rating over time.
Quicksilver 25W40 oil is available in both mineral based and synthetic blend versions. Where possible, I would use the latter for better SAE stability between services.

For Olds and Simplex engines, I suggest using Quicksilver SAE 10W30 oil for all climates, especially as Olds engines have pressure lubrication. The thinner oil on cold-starting reaches the overhead valve rocker gear faster than would a mono grade 30 oil. I do not recommend using semi-synthetic oils in these engines.

If chasing around for a suitable semisynthetic oil for your two-stroke engine is too much trouble, then all two strokes can be operated on 91 RON in conjunction with mineral based oils offered by any of the major oil companies.
Of course, semi-synthetic oils can be used with 91 RON too but, frankly, the benefits of opting for 95 RON are so good. I have been operating my 2005 Tohatsu M8B on 95 RON and semi-synthetic oil since new and the clean running qualities and almost total elimination of spark plug fouling have really spoiled me. Similarly, my 1969 Johnson 1.5 and 1993 Johnson 4 run beautifully smoke-free on 95 RON and XD50 oil.
When it comes to four strokes, the combination of the right fuel and oil make a massive difference to engine longevity. Treat your engine as you would a close mate and not just an object and it will repay you with reliable operation.
A bit of TLC never goes astray!

Andrew Norton

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