Not mentioned so far. W at the front of the number eg. W80 or W100 is for WASHING. 80 or 100 (without W!) is straight oil.
You should discuss with your engineer what oil you are using - straight or washing. You may need to use straight after a rebuild or overhall. If straight has been used for sometime then it may not be a good idea to then switch to using washing.
DISCLAIMER, I am NOT an engineer, just owner. This as I understand it applies to 80 100 etc. Not sure about the multi-grades. You should discuss what you are using with your engineer as swapping, changing, mixing could cost you!!