The best method for filling up a reservoir...

Kevin;
Here, all hydraulic reservoirs over 100 ltrs are filled by using a pneumatic pump out of 205 ltr drum through a quick connect and then it passes through an inline filter of 10 micron filter on its way to the reservoir.
Each of our reservoirs has its own filter for filling.
We also have a counter on the pump so that we know how much is being pumped.

We prefer to fit our OilSafe jugs with the Utility Lids and Hand Pumps. The pump hose can be modified so that it terminates in with a hydraulic quick connect fitting which would match those installed on the equipment. An alternative is to keep a quick connect attached to a short section of hose and a hose clamp in your lube tech tool kit to attach to the end of the pump hose nozzle when you need it. Sometimes just teh nozzle is the best option. We generally do not use the pour spouts because they are more awkward to get the oil where you want it in many tight installations.

With some of the pumps that screw into the Oilsafe containers, there are problems in keeping out airborne contaminants.
Unless they have changed their design in the last couple of months, the 2 areas of concern are:
1. Where the nozzle locates into the top of the pump, when it is removed it allows contaminants into the container. This point also acts as a breather for the container.
2. Where the pump swivels allows for contaminants to gain access.

Just be aware that by adding different nozzles to the hose, you then reduce the effectiveness of the Oilsafe container, it just becomes a container.

Oilsafe containers are one of the best out there, "IF" used correctly

I agree that there is a couple of design concerns with the Oil Safe system as you mentioned, however, I feel these are minor concerns. the OilSafe system is a huge improvement over the old open top galvanized cans we used to ues. If you take some care to keep your containers clean, stow them with the hoses in the port at the top of the cap and filter your oil prior to use you will be miles ahead of those using the old oil cans.
We cannot eliminate every avenue of contamination but we can minimize the potential for contamination. The Oil Safe System is a reasonable method of minimization if used properly.

For our smaller less critical gearboxes we use the Oil Safe system. For our larger systems we have mounted quick disconnect fittings for draining and filling the oil. On the important equipment we use a portable filter skid to move the oil from the drum into the equipment.