#8 exhaust tappet is the first to get oil so if it isn't and others are-------bad tappet, plugged pushrod, etc-You might be able to fish it out thru dist hole in manifold-however since you have a leaky manifold your going to need to fix that but would suggest you check out the tappet first-

for cylinder noise-you can short out spark to cylinders one by one to see if noise goes away or changes------if # 8 becomes quiet or changes siq amount? probably need to pull pan also-oh wait-oil leak