Your best bet for removing varnish manually is lacquer thinner or acetone. Use only on all-metal surfaces and wear protective gloves when handling them though. Regular kitchen gloves won't last long.

If you're concerned about safely removing sludge, deposits or varnish, use Auto-Rx instead. It will take a long time though to remove the baked-on varnish on splashed surfaces where the oil doesn't flow through directly (valve cover, etc.), but at least it won't worsen anymore. And more importantly you'd be cleaning the parts that matter most: piston rings, bearings, oil passages, cam surfaces, etc.