Under Guardiola, Bayern are currently eight points clear in the German league table and have won the the Bundesliga for both of his previous two seasons in charge by dominating their rivals.
Guardiola's current contract expires next June and Bayern bosses are hoping to persuade the 44-year-old to sign an extension during the winter break, which starts on December 20, with a decision expected to be made before Christmas.
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Bayern are again peerless in Germany and Lehmann says the current team is the strongest the club has ever fielded, having picked up 40 out of a possible 42 points with 13 wins from 14 league matches plus a single draw.
"The side which won the treble (in 2013) under Jupp Heynckes was already amazing, but Pep has taken Bayern to the next level," Lehmann told German daily Bild.
"They should set themselves the goal of being the first team to go through a German league season unbeaten."
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Lehmann said Guardiola signing an extension with Bayern would be bad for the rest of the league.
"It would be good for Bayern, because finding an adequate successor would be tricky," said Lehmann.
"For the rest of the league it would be better when he goes and perhaps it will become a bit more exciting."
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