Have not had to bother putting heavier recoil springs in any of my Tanfoglio 9mm's. The original Tanfoglio Springs, while rated at 14 lb's tend to behave more like 15 lb's. 16 lb's is as heavy as I would tend to go and I use that weight in my older .45 Witness. Mainspring wear has to factor into that choice as well. On my newer 2004 vintage .45 Witness, the original Factory Spring has been entirely adequate. Since your FIE Mod TZ75 Custom 90 has had so little use, I would not bother to replace it's recoil spring, as yet. IMO, an 18 lb Spring would keep the action locked up too long, transferring more recoil to the shooter, unless the mainspring has been lightened. While I do have an 18 lb Recoil Spring on hand, the only time it gets installed is when I've used +P .45 ACP.