gap for plugs - MSD box & dist.

When formulating Chevron engine oils, I did a field test of a API SF oil (current classification in 1970's) vs SH oils including Mobil 1 Synthetic. The SF oil had half the additives of the modern formulations. The field test was in NY City Taxi Cabs for 100,000 miles. The SH oils used extended drains of 10,000 to stress the oil and see differences. The SF oil used a 3,000 mile drain. At the end of 100K, the engines were disassembled and rated for wear, sludge, varnish and deposits. The SF oil with shorter drain interval showed less than halv the wear and deposits of the modern oils including Mobil 1. This showed that it is far more important to change oil than worry about the quality of the oil used.
Synthetics don't actually have better wear and deposit control than modern API SN oils as all use high quality base oil to formulate the finished lubricant.
But this is an Ignition thread, not a lubricant thread. Sorry for the diversion.