While an Owner’s decision to add sea-foam …or any other after-market additive….on some imagined schedule that Owner believes to be beneficial… might make that person have a nice, warm, fuzzy feeling he’s done something good for his engine…..In Actual FAct…all that owner has done is UPSET the known-and-preferred composition/make-up of the lubrication-system.
I would replace the word "upset" with "changed". Upset has a negative connotation and it's not a fact that adding something to the engine oil is deleterious. I don't add anything to the crankcase other than oil for any engine I own, but I do put additives into the fuel system of those vehicles. I run Stanadyne in all diesels, and Yamalube fuel stabilizer and Yamalube ring free plus fuel additive in the outboards. I put StaBil in any gasoline that I use in chainsaws, log splitter, concrete mixer, lawn mower, or pressure washer. I only buy ethanol free gasoline for those engines.
Although I haven't seen a diesel engine manufacturer recommend a diesel fuel additive, Yamaha recommends Yamalube fuel additives be used with their engines. They also recommend a 10 micron fuel filter between the fuel tank and the engine.
We need to be honest about manufacturers in general and their recommendations. They will do enough to satisfy government regulations, reduce their liability and get the product through the warranty period, but nothing more.