Engine oil & gear oil

The specifications in the engine manual are decisive: viscosity (SAE), specification (e.g., API/ACEA), and, if applicable, manufacturer approvals. Marine diesel engines are often operated differently than car engines (long periods of inactivity, moisture, partial load), so you should choose an oil that matches the engine type and usage profile. If you provide the engine model, year of manufacture, and operating area, we can quickly narrow down the selection.

Transmission and drive oils are often specified separately (frequently with different viscosities and additive packages than engine oil). The type and manufacturer of the transmission/drive system, as well as the required specification (e.g., specific SAE or ATF grades), are crucial. Do not use an oil that “just seems to fit”—the wrong oils can negatively affect shifting performance and cause wear.

Warning signs include a milky cloudiness (water in the oil), a noticeable odor (fuel contamination), significantly dark/sooty oil (sometimes normal in diesel engines, but critical if there is significant degradation), metal abrasion in the filter/on magnets, and oil consumption. If there is water in the transmission or Saildrive oil, you should act immediately (check the seal/shaft seal), as otherwise there is a risk of consequential damage.

You should only mix in an emergency and only if the specifications and approvals are compatible. A viscosity change (e.g., 10W-40 to 15W-40) should follow the manual and be suitable for the temperature range. Boat engines are a poor place for permanent mixing or “experimentation”—stick to the manufacturer’s specifications.

The following have proven useful: a drain pump (for many inboard engines), a compatible oil filter, gaskets/O-rings, a sturdy collection container, rags/oil-absorbent cloths, and gloves. Bringing the engine up to operating temperature makes it easier to drain the oil. Afterward, adjust the oil level correctly, check for leaks, and dispose of the used oil properly.

Store in a cool, dry, and frost-free location; keep containers tightly sealed and protect from direct sunlight. Seal opened containers securely to prevent moisture or dirt from entering. Follow safety and disposal instructions; never allow oil to enter the bilge, ground, or water.

In most cases, they are not necessary: high-quality oils already contain additives. In some cases, additives can disrupt the additive balance. Use additives only if the engine or oil manufacturer expressly approves them or if a specific problem (e.g., diesel sludge/service life) needs to be addressed with a product designed for that purpose.