Hi Daz, 

What are the models of the Ferris mowers that you have?

Kind regards,

Tate

Hi Daz, 

For the engine oil in a Ferris IS3200Z the information provided to us shows the manufacturer requesting the use of an API SF specification oil in a 5W-30, 10W-30 or an SAE 30 viscosity. With this information we recommend the use of our Small Engine 4 Stroke SAE 30 https://www.penriteoil.com.au/products/small-engine-4-stroke-sae-30-mineral , we have no information regarding the use of a 15W-50 being required for this model. 

If they have recommended you to use a 15W-50 synthetic oil consider the use of our 10 Tenths Racing 15W-50 http://www.penriteoil.com.au/products/10-tenths-racing-15w-50-100-pao-ester

Kind Regards,

Tate