This list is neat, i had to move to digest last evening....

First let me introduced myself.... I do not have a bus yet.... but joined to
help a friend. I have some other exotic vehicle with over 250,000 miles
(original engine) and have done a lot of the maintenance work myself...

I had some valve clatter due to a bad hydraulic lifter once... and i agree
that oil filter can make a change... i have been using fram PH11, and went
to a better NAPA ... the engine had more clatter in the first 10-15 sec like
there was oil starvation... Returned to cheap fram. But 20 minutes is a long
time for the noise to go away. Are you sure your oil pressure is good... you
may want to check with a mechanical gauge!

Also i have been away from cheap oil,  engine parts and labor are expensive
enough to warrant the use of a better oil. I am currently switching to part
synthetic.... 20W50, i do not have oil leak yet (knock on wood) because i
replaced the oil seals, as the drip was too much. I cannot tell any
difference... and prolly will not, it is just extra protection for hot days
or slow traffic.

Michel