Most engine oils contain zinc in their mixture, although the level of zinc has been decreased since catalytic converters became common on cars. The zinc was used to help "repair" minor wear in steel parts, and is very effective at that job. However, it also causes damage to catalytic converters, which is why the concentration of zinc is less now than in 1970. It is widely believed that the reduced amount of zinc in engine oil has caused premature wear of engine components in pre-catalytic car engines that were designed with the expectation that high levels of zinc would be in the available engine oil.
Most diesel oils have higher levels of zinc due to the higher loads found in diesel valve trains. I use Mobil Delvac in both my Jetta diesel and my motorcycle. I do not have any empirical evidence of the benefits of zinc in our motorcycle engines, but I know without a doubt that low zinc oils will allow specific high-load valve train components in the VW diesel engines to wear out VERY quickly. If you run a low zinc oil in a post 1990 VW diesel engine, then your valve train will wear out within 200,000 miles. If you run a high zinc oil in a post 1990 VW diesel engine, then you will likely never have to replace your valve train.
There seems to be a threshold at which increasing the amount of zinc in the oil has no further benefit. I am not sure what that threshold is, but the mobil Delvac I use seems to be good enough for my diesel, so I am using it as is for my motorcycle (it comes with HondaMan's blessing too). If you are using a low zinc oil, then you might see a benefit to adding the zinc additive... or you could just use mobile Delvac (usually on sale for $9/gallon at my local O'Reilly).
Camelman
