Purolator & Wix will not warranty standpipe oil filters for /6 but Baldwin will.

A filter may fit, but if the manufacturer doesn't include it in their current catalog for a certain application, they won't likely warranty it. Even if it fails due to manufacturing defect, they may deny a claim because of mis-application. In the case of an engine that hasn't been produced in 30 years and may have issues with a certain filter, just not listing an application is the safe way out.

Filter companies know they will have at-fault failures, and build the cost of them into the price of the product. This is M+H's way of reducing liability. Good thing there is an alternative. Baldwin's are a very good filter.