I've been shooting Olympus for years, and the OM-1 since it came out (pre-ordered). I didn't see this thread in time to offer buying advice, but I'm glad you found something that works for you (100-400mm). Perhaps some months later, you might want to view your photos and see how many shots were shot at under 300mm. My suspicion is that there would be few of them, and that you will find the 300mm Pro + MC-20 (600mm) far more rewarding to use.
Even with the 2x teleconverter, the sharpness of that lens is insane. It's manufactured to very high standards and I've never heard anyone complain of a poor copy even with a used lens, which are quite affordable these days. That's what I use anyway, after having tried both the Olympus / Panasonic 100-400's and some other systems e.g. Nikon 200-500mm and D500.
Does anyone recommend lens filters these days ? If so what would be a good brand ? TIA 😊
Regarding filters: the primary purpose of one, IMO, is to protect the coating on the front element of your lens from scratches, unless you want a CPL filter or a special effects one. Replacing a filter is a lot cheaper than sending a lens in for repair. With a high quality (expensive) filter, the degradation is minimal and you'd barely notice it. You want to use a multi-coated brass filter. As with high-quality optics, you can safely buy a used filter and be assured of good quality, so long as it's manufactured to high standards.
I use a
B+W XS-Pro 007 Clear MRC Nano filter with my 300mm Pro, and do not bother with filters for my cheaper lenses. They might have a newer equivalent model now.
007 indicates that the glass is thinner than their 010 filter (less impact on image quality). You want a Clear or a UV filter for birding. MRC means that it is multi-coated to minimize aberrations and light loss. Nano is their water-repellent coating for bad weather.
Other brands might have other nomenclatures, but the key factors remain the same: brass construction and multi-coating. A good brand would include charts of light loss in their literature.