I was talking to a Mobil tech over the phone not long ago about cleaning up heavy varnish using Mobil 1 0W-40. I asked if shorter intervals were necessary. His recommendation was to use a quality filter and following the manufacturer's recommended intervals or the Oil Life System, and over time it would be cleaned up.
I also spoke to Liqui Moly's R&D Manager over heavy varnish build up in the cylinder head area, and Doug Hillary before coming up with a plan for myself for my Mitsubishi 6G72.
As it stands, I'm running Mobil 1 0W-40 at 3,000 mile intervals and running Liqui Moly Pro-Line Engine Flush for 25 minutes before draining it out. The LM R&D Manager suggested 25 minutes over the standard 10 minutes and to not run the stuff for more than 25 minutes. It appears to be been working quite well for me so far, and I will be pulling the covers off after about 50,000 km (31,250 miles) to see what the internals are like after 10 attempts.
Liqui Moly recommend 100 ml/L for their Pro-Line Engine Flush, so 700 ml would be enough for an LL8. Personally though, I slightly overdose and run 2 cans (1 can is 500 ml) in my own LL8.
Make a long story short, run a quality cleaning oil with low NOACK - Mobil 1 0W-40/5W-30 ESP or Pennzoil Ultra for example, and use a quality flush for slightly longer. Amsoil's stuff is also pretty good.
