The best replacement for 15w40 is 20w50.

The best alternatives to 15w40 are rated as following:
★★★★★  20w50
★★★★★  10w30
★★★★★  10w40
★★★★★  5w40
★★★  5w30
★★★  15w50
★★★  20w40
★★  sae 40
★★  15w30
★  0w40
★  sae 30
★  25w40
★  5w20
★  10w40 oil
★  20w50 oil
  40w
  10w50
  5w40 synthetic
  10w30 oil
  5w50

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Speaking about 15w40, one probably shouldn't forget about 10w40.

Other search terms related to 15w40 and their ratings are:
★★★★★ 10w40
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15w40
20w50
10w30