That manual was written 25 years ago. Oil today is light years ahead. However if it is showing signs of sludge I suggest confirming the lower intake gasket isn't leaking. Otherwise you're wasting your time putting a high grade,elevated cost oil in it.
Once you've confirmed its properly sealed I suggest sending Dermapaint a message. He had one and if I know anything about him I know that his innards were spotless once he acquired it,though it wasn't prior to him maintaining it.
As him what he did and mimic it. He's a sensible guy so you can rest assured whatever he did was cost effective and got results.
You'll get opinions from all different members minds,and most are sensible. However I'm not one who is hyper sensitive when it comes to additive use. I feel some are effective and perform as advertised.
Personally I'd feel most confident with rislone,based on Molekule recommending it a number of times,and iirc the only additive I've ever seen him recommend.
Knowing that it's what I recommend to an additive wary consumer who isn't confident,nor experienced with other cleaning,solvent based additives.
And I'd stick to the suggested 3000 mile interval however I'd use whatever conventional you've got in the stash or happens to be on sale when you buy it.
Rislone won't clean overnight however it will definitely clean things up,and no rational concerns in relation to solvent thinned oil in the sump an entire interval which in all honestly will flash off leaving the carrier oil behind and that carrier oil gets flashed off resulting in the oil you started with and a bunch of dissolved deposits.
If using a mild solvent flush like MMO or Seafoam I use them at the beginning of the interval,using an inexpensive conventional and I'd change it based on the blotter test which could be as little as a thousand miles,or possibly the whole 3000.
Play it by ear. Decide the route you're most comfortable with and once you've got a set plan use the tools that will be the most consistent with you're chosen set of parameters.
Varnish isn't a big deal in my opinion,and truth be known slight deposit accumulation won't affect automobile performance so there does come a point where there is no real gain in spending funds trying to clean up a worn engine,it's smarter to stop spending any money on anything more than the most basic maintenance and fuel.
However I do feel that if a vehicle runs well,has no significant flaws and can be expected to remain a dependable appliance to get me where I need to be,then it is prudent and smart to get the drivetrain into tip top shape and potentially saving a large expenditure down the road.
I like motor oil saver now for cleaning up an engine. Harmless,conditions older seals,elevates TBN which is beneficial in a higher mileage engine where more combustion gasses tend to escape the combustion chamber. And it's cost effective.
It's not going to clean it up over night however it will clean it up. Sometimes it's better to take a slower route but still achieve you're goal rather than try a potentially harmful approach and have it blow up in you're face.
Just my opinion.