If you have that sludge it may not have been lubricated properly for a bit. I flush the crankcase. I like to use diesel, fill the crankcase to proper level, then lean the bike back and forth pick it up shake it a bit and then drain it out. Put another load of diesel in it and run it for just a little bit. After this clean your oil filter and put fresh oil in it. Did you double check to make sure the 4 hole oil filter was installed?Forget to mention there was a strange sludge on the underside of the oil filter, consistency of mud. Can’t be a good sign.