The company that produced the forklift can deny warranty for use of improper weights of oils, hydraulic or engine, transmission, differential. If you are not the dealer it would be wise to obtain a specification sheet and stick to the weights and codes. The dealer may refuse to act but it would not be in their best interests. Most dealers I have dealt with will come check the unit and if the tech cannot see abuse or determine abuse, they file a warranty claim. It is up to the factory whether warranty is paid or not. If the factory denies the dealers warranty claim, the customer will be responsible for the cost of repairs. Many warranties do not cover the cost of fluids. Hyster corp. sent their dealers a letter about using the wrong viscosity of oil in engines and voiding said warranty. Under normal conditions, I don't see the factory refusing warranty, and would severely reprimand a dealer who refuses to check on a warranty issue. To many major accounts for any lift truck company do their own work for this to be an issue.