Imo, I find SETTING timing accurately through that window is a VERY hard thing to do. I remove the primary cover, mark the alternator at 0 TDC, and also mark the rotor where I want to set the timing.(You can reference/use the marks on the flywheel thru the window to mark the alternator rotor),( I use a degree wheel.) Any time you want to check or set your timing, it's just a matter of draining the primary, remove outer cover, and check/set the timing with your timing light. Much easier, than trying to see thru the oily hole. Unless you have "TINKERITESS" you should be able to time the engine 1 time, and forget it till the next modification.
John