Polytetrahydrobenzoxazines and bistetrahydrobenzoxazines, obtainable by (A) reacting at least one diamine of the formula H2N-A-NH2 with a C1- to C12-aldehyde and a C1- to C8-alkanol at 20 to 80° C. with elimination and removal of water, (B) reacting the condensation product from (A) with a phenol which bears a long-chain substituent at 30 to 120° C., and optionally (C) heating the reaction product from (B) to 125 to 280° C. The resulting polytetrahydrobenzoxazines and bistetrahydrobenzoxazines are suitable as fuel or lubricant additives, especially as detergent additives for diesel fuels.