Unhappy client with after sales service

Good day. 3 months back I purchase a petrol trim tech lawn mower from builders wearhouse(with a warrentee) the suplier of this product is trim tech (tandem) the lawn mower did not want to start at all so i took it back.builders wearhouse send it to the suplier to fix it. They said i used the wrong oil(which is not the case) and they said i must pay R3000 to replace the engine the wont pay.i only used the oil that was suggested by the builders wearhouse attended when i bought this product and it was also trim tech oil?i stringly advise people tot to buy this product.