Lewis (3-4) turned in his third straight quality start, yielding five hits and only one walk. The sole run he allowed came on Cliff Pennington's bases-empty homer in the sixth inning.
Beltre's two-run blast in the fifth gave the Rangers a 4-0 lead and Mike Napoli added a run-scoring double later in the frame, putting Texas comfortably ahead.
Michael Young went 3-for-4 with two RBI and two runs scored or the Rangers, who improved to 3-7 in May.
Brett Anderson (2-3) was touched for four runs on six hits, lasting only 4 1/2 innings for the A's.