Last reviewed: 2026-05-07 > Quick answer: Mercury's current 4-stroke outboards run on 87 octane regular gasoline. Premium isn't required and doesn't help. The bigger fuel issue is ethanol: E10 pump gas absorbs water, phase-separates in storage, and gums fuel systems. Use...
Last reviewed: 2026-05-07
Quick answer: Mercury's current 4-stroke outboards run on 87 octane regular gasoline. Premium isn't required and doesn't help. The bigger fuel issue is ethanol: E10 pump gas absorbs water, phase-separates in storage, and gums fuel systems. Use ethanol-free 89 marine gas where available (HBW sells it at the dock), or run fuel stabilizer in every tank if you cannot avoid E10. Older supercharged inline-6 Verados require 91 octane premium.
You're standing at the pump. The motor ran fine last fall. It's been sitting all winter and you want to fill it up and get on the water.
Regular, mid-grade, or premium? What's this E10 label mean?
Nobody at the gas station is going to help you figure this out. So here it is in plain English.
The Ethanol Problem in Plain English
Most Ontario pump gas, including the 87-octane "Regular" you've been putting in for years, is E10. That means 10% ethanol, 90% gasoline.
Ethanol absorbs moisture from the air. In a car, that's not a big deal: you burn through a full tank in a few days. The water never has a chance to settle.
In a boat, you might fill up on the May long weekend and the tank sits for three weeks before you're back on Rice Lake in the Kawarthas (Ontario). Or longer. Across a full season, that moisture accumulation adds up. Ethanol pulls water from humid air right through the fuel cap, through the vented lines, and into your tank.
Here's what happens next: the water and ethanol eventually separate from the gasoline, a process called phase separation. That water-ethanol mixture sinks to the bottom of the tank, where the fuel pickup sits. So the first thing your motor draws when you try to start it is water, not fuel.
The result: hard starting, rough running, corrosion inside the tank and fuel lines, gummed-up carburetors, and service bills you didn't budget for. It's one of the most common reasons boats come into the shop needing fuel system work after storage.
Why Boats Are Way More Vulnerable Than Cars
Your car doesn't sit. Even if you take a two-week vacation, the tank cycles through in days once you're back. Ethanol's water-absorption issue is largely self-correcting in a car.
Boats are different. A fuel system that sits for two to six months with E10 in it is a fuel system in slow-motion damage mode:
- Fuel lines and primer bulbs made from rubber compounds that were designed for straight gasoline can swell, crack, and get soft from ethanol exposure. A primer bulb that won't pump firm is often an early sign.
- Carburetors on older and smaller motors are especially vulnerable. The varnish that E10 leaves behind when it degrades can clog jets and needles to the point where the carb needs to come apart and be cleaned before the engine will run properly.
- Gaskets and O-rings inside carbs and fuel pumps can deteriorate, leading to air leaks, lean running conditions, and stumbling at wide-open throttle.
- Steel fuel tanks can corrode from the inside when water sits at the bottom. Aluminum tanks are more resistant but not immune.
Our techs see this every spring and fall: motor ran fine when it was put away, now it won't start or bogs the moment you push the throttle past half. The Mercury Outboard Won't Start troubleshooting guide covers many of these symptoms, a significant portion trace back to the fuel system.
What Octane Your Mercury Actually Needs
Octane measures detonation resistance, how well fuel resists igniting before the spark plug fires. Higher octane doesn't mean more energy, cleaner running, or more power. What it does is prevent knock in engines designed for higher compression. Put it in an engine that doesn't need it and you've spent extra money for nothing.
Here's the general picture for Mercury outboards:
Most 4-Stroke Mercury Outboards (Under 250 HP)
The vast majority of modern Mercury 4-stroke outboards are engineered for 87 octane (R+M)/2 minimum, the standard pump Regular. Running 91 doesn't help them. Running 87 is the right call.
Verado and High-Output Pro XS V8 Models
Mercury's Verado line and certain high-output Pro XS V8 engines are higher-compression motors that require a minimum of 89 or 91 octane, check the engine sticker or your owner's manual for the specific requirement on your motor. Don't guess. The sticker is there for a reason.
The Rules in Short
- Lower octane than required = detonation/knock = engine damage over time
- Higher octane than required = wasted money, no benefit
- When in doubt: read the sticker on your motor, not the marketing copy on the pump
The Mercury Outboard Fuel Efficiency Guide covers prop selection and RPM tuning in detail, the performance side of the equation this post doesn't.
The Ethanol-Free Advantage
Ethanol-free gasoline, pure gasoline with no alcohol blended in, doesn't have the water-absorption problem. That changes the equation significantly for boats.
Here's what ethanol-free does for you:
Storage life. E10 fuel starts degrading in as little as 30 days, especially in heat. Ethanol-free gasoline holds up for six months or more without a stabilizer. For seasonal boaters, that window matters.
No phase separation. Without ethanol, there's nothing to attract and absorb moisture. The water-separation risk disappears.
Carbureted motors run better on it. Older motors and small kickers designed before ethanol blends were common run more reliably on ethanol-free. Many outboard manufacturers specifically recommend it.
Slightly better fuel economy. Ethanol contains less energy per gallon than gasoline. At a 10% blend the effect is modest, but it's real.
Longer fuel system life. Without ethanol attacking rubber compounds over time, lines, primer bulbs, and carb internals last longer.
HBW Sells Ethanol-Free Fuel On-Site
You don't have to find a specialty station or drive out of your way. Harris Boat Works carries ethanol-free fuel at the dock. Fill up on-site, right before you launch or while you're tied up, no trailer required.
Call 905-342-2153 for fuel hours before you come in.
What About Premium Pump Gas (91/93)?
This is where a lot of people make an incorrect assumption: premium pump gas in Ontario is typically still E10 unless the pump label specifically says otherwise.
"Premium" and "ethanol-free" are not the same thing. Premium refers to octane rating only. If you're buying premium at a regular gas station, read the pump label. If it doesn't explicitly say "ethanol-free," "no ethanol," or "non-ethanol blend," assume it contains ethanol.
Don't pay premium prices thinking you're protecting your fuel system. Read the label.
Fuel Additives and Stabilizers: When You Actually Need Them
Stabilizers work by slowing the oxidation and degradation of fuel. Here's when to use them and how:
Any storage over 30 days, use a stabilizer. This applies even with ethanol-free fuel, though the urgency is lower. With E10, 30 days is the real threshold before fuel quality starts dropping.
Proven products: Sta-Bil 360 Marine, Mercury Quickare, and Mercury Quickleen are all well-regarded in the field. Sta-Bil 360 Marine is specifically formulated for marine applications and handles the ethanol/water issue directly. The others are good for general fuel system maintenance and carb cleaning.
How to add stabilizer correctly: Add it to the tank, top off with fuel so it mixes in, then run the motor for 10–15 minutes. That circulates the treated fuel all the way through the system, carb, fuel lines, the works. Shut down and store. If you skip the run step, the carb bowls still have untreated fuel sitting in them.
Don't overdose. Follow label directions. More is not better.
The DIY Mercury Outboard Winterization Guide walks through the full storage prep sequence.
Mid-Season Fuel Hygiene Tips
Fuel care isn't just a spring-and-fall task. A few habits during the season pay off:
Top off the tank before leaving the dock. Less air space means less condensation surface area.
Use a fuel-water separating filter, and change it every season. Last line of defense before water reaches your motor. Milky filter bowl = moisture problem.
Check your fuel lines every spring. Ethanol-damaged lines feel soft or tacky. A line that looks fine on the outside can be breaking down from the inside.
If the boat sat with E10 through winter, don't just top up with fresh fuel. Diluting old, phase-separated fuel doesn't fix the problem. Drain it, inspect the separator, and start fresh.
The Spring Outboard Commissioning Checklist covers this step by step.
Where to Fuel Up on Rice Lake, Ontario (in the Kawarthas)
For Rice Lake boaters, Harris Boat Works has on-water fuel access, pull up to the dock, fill up, get back out. Ethanol-free available on-site. No need to trailer to a gas station.
For fuel hours, call 905-342-2153 before heading in.
Common Fuel-Related Symptoms HBW Techs See Every Season
If your motor shows any of these, a fuel system issue is likely somewhere in the chain:
- Hard starting after storage, especially if it ran fine when put away
- Engine bogs at wide-open throttle, motor pulls fine off idle but falls flat at full throttle
- Primer bulb won't pump firm, stays soft even after pumping; often a bulb or line issue from ethanol degradation
- Varnish in the carb bowl, yellow or orange residue; degraded fuel left behind
- Water in the fuel-water separator, phase-separated or condensation moisture made it through
- Fuel gauge reading inconsistently, can indicate a sender issue or internal tank corrosion affecting the float
Some are straightforward fixes. Some need a full carb clean or fuel system inspection. Catching them early is cheaper than a mid-season breakdown on the water.
The Yearly Fuel Calendar
A simple seasonal checklist that covers the full year:
Spring (May)
- Inspect the fuel-water separator, replace the filter element if it's been more than one season
- If you stored with E10 and didn't stabilize, drain the tank and start fresh with ethanol-free
- If you stored properly with stabilizer and ethanol-free, top off with fresh ethanol-free
- Run the motor briefly at the dock before launching; confirm fuel flow and no hesitation
Summer (June–August)
- Top off the tank at the end of each trip or before multi-day sits
- Inspect the primer bulb and fuel lines periodically, ethanol damage shows up gradually
- Check the fuel-water separator bowl for water if you're running E10
Fall (September–October)
- Add stabilizer to the tank, top off, run the motor 10–15 minutes to circulate treated fuel through the system
- If storing indoors, a full tank reduces condensation; if storing outdoors, check manufacturer guidance
- Note the condition of lines and primer bulb, order replacements now if anything looks suspect, not in May when everyone else is calling at the same time
Winter (November–April)
- No fuel system action needed, the storage prep does its job
- If you didn't stabilize properly, note it now so you can address it in spring before everyone else calls at the same time
Quick Decision Card
| Situation |
Fuel to Use |
| Most Mercury 4-stroke outboards (under 250 HP) |
Ethanol-free 87 octane |
| Verado / high-output Pro XS V8 |
Ethanol-free 91 octane (check engine sticker) |
| Storing more than 30 days (any fuel) |
Add marine fuel stabilizer |
| Unsure of your motor's octane requirement |
Check the engine sticker or owner's manual, it's there |
| Buying "premium" at a regular pump |
Read the label, premium ≠ ethanol-free in Ontario |
Need Ethanol-Free Fuel or a Fuel System Service?
We have ethanol-free at the dock and we diagnose fuel system issues every day.
For service requests (fuel system inspection, carb cleaning, fuel line replacement, seasonal commissioning): hbw.wiki/service
For fuel and fuel hours: Call 905-342-2153
Harris Boat Works. Gores Landing, ON, est. 1947