I may have to disagree on the amount thing. Silica gel, unless near expanded, will continue to take in vapour, thus the humidity within the box will drop significantly low regardless of amount eventually. The larger the gel spread area, the faster the intake.
On the lubricant drying out, I am skeptic about this as my impression is, synthetic grease dries faster under heat and open air, not dryness. Although I cannot say it is confirmed but I have never encounter such issue with silica gel.
I know of paper, real leather and natural rubber becoming brittle when store in too dry of a condition. But again for camera equipment, such encounter are only exhibited when it item is buried or direct contract with silica gel.