What Happens If You Put Synthetic Oil In A Weed Trimmer?

These days, corded and cordless string trimmers are tough enough for most home maintenance tasks, but when it's time to clear heavy brush or tackle dense, overgrown grass, you really need a gas model. Whenever you use a tool with a gas engine, you have to give it oil, and that begs the question of whether you can use synthetic oil with such a small engine. The answer, say mechanics and lawn care professionals posting on Quora, is yes. Considering the superiority of synthetic oil, good things are more likely to happen than bad things.

Among the many advantages of synthetic oil are that it lasts longer, degrades more slowly, and has less tendency to evaporate and get lost through the engine exhaust system than conventional oil. Crucially for a trimmer that gets stored over the winter, synthetic oil has lower tendency to thicken and clog the carburetor at low temperatures. Because of this, it's possible the trimmer will start more easily when you give it synthetic oil. 

The main downside to synthetic oil is that it's more expensive. That makes a big difference when you're adding oil to a car, but isn't a major factor when using the small quantities you need for a weed trimmer. It's important to realize that you should avoid using the same oil in your trimmer that you use for your car. Synthetic oil specifically formulated for a small engine — either two-stroke or four-stroke, depending on the trimmer model — is the way to go.

Now that you know that you can use synthetic oil in a weed trimmer, the next question is: "Should you?" The answer is that it probably doesn't matter, because important advantages of synthetic oil are lost on a small engine. It will lubricate as well as conventional oil, but not necessarily better. However, you may want to make the switch if your trimmer is hard to start after a period of sitting idle, because synthetic oil can help with that.

The kind of oil you use, and the way you add it, differ depending on whether your trimmer has a two-stroke or four-stroke engine. Four-stroke engines have a separate oil reservoir, and you pour four-stroke oil directly into that. Two-stroke engines have no reservoir and take a fuel/oil mixture with two-stroke oil. You should never use four-stroke oil for a two-stroke engine, and vice versa. The gas-to-oil ratio for a two-stroke engine is typically 50:1, but some engines take a 40:1 mixture, while others take 32:1. Always check the manufacturer's specifications. 

You might be wondering whether you can add synthetic oil to your trimmer if the reservoir or gas mixture already contains conventional oil, and the answer is yes. Synthetic oil is just a modified form of conventional oil, and there's no danger of a chemical reaction if you combine them. The only drawback is that diluting synthetic oil in this way negates some of its benefits.