[hogcowboy]

Both oil and plugs have made major advancements since 1988 and the printing of your owners manual. If you are using the exact same plugs and oil then I'd continue as you are. But if using an improved oil or plug I'd up the interval slightly. Depending on oil, 5k-7k might be okay and plugs could be 10k-20k. Some plugs can go a lot more but doubt you'd want to use them simply because of cost. I certainly hope you are changing the oil filter as well. It too might be an improved version. Things have changed. Need to change with it sometimes.