Goldrush,
I find that when cleaning up an old engine with very dirty oil.
I do the following, drain out the old oil. Refill with "Kero' ,disconnect spark plug wire.
Pull it over about 3 times a day for a couple of days, drain and repeat.
When Kero looks clean, refill with oil, pull the engine over a few times, pull plug and put a little oil into spark plug hole, replace plug, pull over a few more times.
Attach plug wire and fire her up and let run till very warm. shut down, check oil for color. If clean, replace with SYN!! If not change oil again.
Fred
Husqvarna STE927(11.5HP) snowblower, MTD Pro Series 18/42 Lawnmower, MTD 6.5 HP Self Prop Lawn Mower, Weedeater 1500 Blower, Web Gensis 2000