When the sound immediately preceding Allah’s name has fathah or dammah, the lam of that name is read heavily (with tafkhim). The ruling is governed by the preceding pronunciation rather than by treating every lam alike.
«قَالَ اللَّهُ» — the preceding sound has fathah, so lam is heavy.
«عَبْدُ اللَّهِ» — the preceding sound has dammah, so lam is heavy.
Heaviness does not transform lam into ط or ظ. Its original articulation remains intact; only the required resonance is added. Avoid excessive pressure, lengthening, or an artificial change in the consonant.