You can fill it by taking off the derby cover.

 

I used the same gasket on my last bike for at least 5 oil changes. I changed the primary and tranny every 10,000 miles. Engine at 5,000. Never leaked. It was an '09 (no inspection cover) and I used the old style gasket instead of the o-ring since I changed derby covers. That little funnel Harley sells to fill through the Derby cover opening really makes it easy.

 

On my 11, I reused the frisby gasket for two years and 30K miles of 5k interval primary changes, never leaked.

 

I too have an 88" motor (actually a 95" now). The primary is best filled through the derby cover (as recommended by the shop manual).
Although re-filling through the 'inspection cover' (the one for adjusting the primary chain tensioner) would not be entirely impossible, it would be very awkward since it is well below the level of the oil. It would require laying the bike nearly on it's side.
I agree that an inspection cover on the later motors would be a welcome addition. It would be pretty handy for checking the automatic ramp tensioner.

 

Glad you agree.

 

I don't know what bike you've got, but the bottom of the inspection cover is about the same height as the derby cover on early TC FLT "short" primaries.. The oil can be filled through either opening and neither will spill even with the bike resting on its side stand. (About 1 quart on an '04 if memory serves.)

 

I can never tell where the level is so I just put in the recommended amount per manual. I have used the derby gasket several time without replacing. The inspection hole it just as easy to fill the primary, but also has a gasket.

 

My bike is an '06 Street Glide.
Problem is, the bottom of the inspection cover is 3/8" lower than the bottom of the derby cover. With the bike on it's side stand, primary oil spills down on the floor if it is not drained before removing the inspection cover. (I found this out the first time I tried to adjust the tensioner without first draining the primary.)
So....re-filling through this cover isn't impossible, just unnecessarily difficult when the derby cover is so very easy to use (and spelled out in the shop manual as the correct method).
And with that reply I think this horse is now quite dead.

 

Ive always filled mine through the inspection cover. Throw a quart in it and button it up.

 

I change the gasket maybe every 3rd or 4th time. I've never had one leak before actually.