mmm, a dax rush with a cossie engine - yummy

Castrol 10/60 is what many people use, certainly for older engines, anything thinner will just go out in smoke through your exhaust.

Could just be a coincidence that it ran on three after a wash to be honest. I had a similar problem on mine (running on three), turned out to be a plug which had a hairline crack on the ceramic insulator.
There are many other things that could be the reason it's running on three. I assume you know what to look for.

The intermittent tap could be a sticky follower, they get gunked up with time and short or replacing or cleaning them, there isn't much you can do.