Evolution of Li surface abundance as a function of effective temperature for models at different masses (0.8 − 1.5 M_⊙, colour-coded) and solar metallicity. Models including a constant viscosity (ν_add = 3.5 × 10⁴ cm² s⁻¹) and a time-dependent viscosity (as defined in Sect. 4.3) are represented in full line and dashed-line, respectively. Top panel: main sequence. Bottom panel: SGB and RGB phases. The standard Li-rich giant threshold of A(Li) = 1.5 dex is indicated by the grey dashed-line. Observational data are from Luck & Heiter (2007), Pace et al. (2012), Carlos & Meléndez (2020), Semenova et al. (2020), Mallick et al. (2023) and represented by colour-coded squares. The Sun is represented in yellow with its usual symbol (Asplund et al. 2021).