Can I Use 20W-50 Oil as a Top-Up for 15W-40 Engine Oil?

Yes, you can use 20W-50 oil as a top-up for your 15W-40 engine oil in most cases, but there are a few matters to consider.

Viscosity Differences

Both 15W-40 and 20W-50 oils are multi-grade oils, meaning they have similar base oil properties. However, 20W-50 has a higher viscosity at operating temperatures compared to 15W-40. This difference in viscosity might cause the overall mixture to have a slightly higher viscosity, which may not be ideal for all engines, especially those designed to work with lower viscosity oils. Increased viscosity can lead to more resistance during engine operation, potentially affecting fuel efficiency and power output.

Engine Condition

For older or high-mileage engines, a thicker oil like 20W-50 can sometimes be beneficial. It can provide better sealing and reduce oil consumption, compensating for the wear and tear of long-term use. However, for newer or well-maintained engines, maintaining the original viscosity is crucial to ensure optimal performance and longevity.

Manufacturer Recommendations

Always check your vehicle's owner manual for oil specifications and recommendations. Using oils that are outside the recommended viscosity range, especially in extreme conditions, can lead to various issues. For example, if the oil isn't compatible, it might not lubricate the engine properly, leading to increased wear and potential damage.

Mixing Oils

While it is generally acceptable to mix different brands or types of oil provided they are compatible, mixing different viscosities can lead to unpredictable results. Mixing 15W-40 with 20W-50 can create an oil with an inconsistent viscosity, which could cause problems during engine operation.

Additional Considerations for Two-Stroke vs. Four-Stroke Engines

For a 49 cc two-stroke scooter, a thick oil like 20W-50 might not be ideal. In two-stroke engines, the oil needs to work only in the crankcase. While a thicker oil may not be a problem due to the relatively small capacity of the crankcase, adding 20W-50 might create a drag, making the engine feel less responsive during operation. This can be particularly noticeable when compared to a more fluid 20W-40 or even a lighter viscosity like 10W-40.

In my experience, I once used 20W-50 GTX in an 80 cc two-stroke scooter, and it resulted in the scooter performing differently. After draining the 20W-50 and topping up with 20W-40, the scooter returned to its original performance. Similarly, for a four-stroke scooter, it is strictly recommended to use the OEM-recommended grade of oil. Opting for 20W-40, 10W-40, or even 10W-30 would be a better choice, but using 20W-50 is not advisable.

Engine Sump Capacity and Recommendations

Adding 1 quart of 20W-50 to a 5-6 quart sump shouldn't cause a significant problem if done only once. However, continuing this practice is not advisable. Engine builders and designers are aware of the importance of specific oil viscosities; hence, their recommendations should be followed.

In some engines, adding 20W-50 may not be an issue, while in others, it could cause problems. It ultimately depends on the type of engine. Therefore, for the best results, always refer to the vehicle's owner manual or consult a professional mechanic to ensure you are using the correct oil for your specific engine.

Overall, while using 20W-50 as a top-up is generally safe, it's essential to consider the viscosity, engine condition, and manufacturer's recommendations to maintain optimal engine performance and longevity.