A friend bought an ex-factory Kawasaki H2R engine and carefully built up a complete near-replica bike around the three-cylinder 750cc two-stroke from his extensive collection of cycle parts. It was a nice-looking machine.

One weekend, he phoned and asked if I would like to go with him to a vintage race weekend at Loudon, New Hampshire, some 90 miles away. He picked me up and we rolled out with motorcycle, parts, and tools in the back of his van. In the course of our conversation, he revealed that this weekend he was trying a new synthetic gear oil.

These days, the word “synthetic” has an almost-hypnotic effect, for it seems to imply that the product is modern and scientific, with vastly improved properties—far better than nasty old-time oils haphazardly refined from petroleum, which comes from word roots literally meaning “rock oil.”

Arriving at the track, we unloaded the motorcycle and my friend’s face fell. Oil was steadily dripping from the gearbox output-shaft seal. The rubber-like polymer in those seals was the standard of its time, 1973, so it was probably Buna-N, a nitrile compound. Oils have different solubilities in seal material, which can lead to dimensional changes, including swelling and shrinking.

My friend’s voice had a desperate edge. “What am I going to do? Do I have to give the weekend a complete miss?”

Clearly it was time for someone to play Mr. Wizard, so I said the only thing that made sense in the time available.

“Drain the synthetic out of the gearbox, and refill it with what you ran before. That may reverse whatever is happening.”

In the media tables published by the Parker seal company, silicone is listed as “unsatisfactory” for gasoline. For water, however, silicone gets a “satisfactory” compatibility rating.

Compatibility between seal materials and fluid media is a case of “horses for courses.” Another friend in another time, about to install a vent valve on the fuel tank of his racebike, found he didn’t have the right size. So he sent a friend to an industrial outlet to “get the best thing you have” in the dimensions needed. The friend came back with a pricey silicone O-ring that didn’t like gasoline at all, and in the race the rider was out with gasoline spraying in his face.

In the media tables published by the Parker seal company, silicone is listed as "unsatisfactory" for gasoline. For water, however, silicone gets a "satisfactory" compatibility rating.

Returning to the case of the dripping gearbox seals, I can report a happy ending. Once again submerged in gear oil it “liked,” the seal stopped leaking, and the weekend at the track was enjoyable.

A sensible rule is to switch to a synthetic lubricant only when petroleum-based oils can no longer perform satisfactorily, not just because you like the sound of “synthetic” better. As an example, early gas-turbine lubricants were petroleum-based and were for a time satisfactory. But as bulk-oil temperatures rose in the next generation of jet engines, petroleum oils could no longer take the heat. The next step was a switch to diester-based turbine oils. When they in turn proved marginal in jet engines that ran hotter yet, oils with the mouth-filling name neopentyl polyol esters were adopted.