I have a 2003 15hp Tohatsu fourstroke. I've changed the oil several times using Nissan oil. But is it really necassary to use the manufactures oil? The ownners manual said that the motor should use SAE 10W-30/40 type oil. The only reason I ask is because of price, "Nissan Marine Premium 4 stroke oil" is $8 per quart, as opposed to $2 a quart for other major brands.

Yes---VERY NECESSARY------You can use other brands of oil, but, It must be 4 stroke outboard motor oil. You paid a lot extra for the 4 stroke why shorten it's life over a few extra buck$.

Four stroke oil for marine motors have additives to help against corrosion due to the environment the motors are used in. Spend the extra money and purchase an approved FCW oil.

Here is the advice directly from Mercury Marine website:

"If Quicksilver or Mercury Precision Lubricants 4-cycle outboard oil is not available, use a premium quality 4-cycle engine oil, certified to meet or exceed any one or a combination of the following American Petroleum Institute (API) Service Classification SH, SG, SF, CF-4, CE, CD, CDII.
Always consult your Operation, Maintenance and Warranty Manual for additional information concerning viscosity requirements and temperature ranges. "

So SG-SH-SF rated oil works just fine.