Grease is simple and cheap. Oil runs cleaner and longer. Here’s when each one wins.

What they do

Both oil and grease seals form a barrier between the moving shaft and the stationary stern tube. The lubricant lowers friction and keeps the sealing elements running smoothly. Beyond that, they differ.

Grease-type (traditional)

A grease cup on deck feeds grease down a line to a sealed stern-tube chamber. Grease the cup periodically; grease pushes out through the shaft seal. Simple and robust, works on any size shaft, but you must remember to grease it.

Oil-type (Super Seal and similar)

A sealed reservoir of marine oil feeds the stern tube. Oil runs cleaner than grease, cools better, and lasts longer between services. More components, more cost, but less daily attention.

Dripless face seals

A separate category: carbon face against a stainless rotor, water-cooled. No grease or oil in the normal sense. Longest-lasting of the three but needs well-aligned, straight shafts.

Which to pick

Casual pleasure boat: grease is fine, cheap, forgiving. Commercial / high-hours: oil-type pays back in less service time. Modern pleasure boat where bilge dryness matters: dripless face seal.

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