My boat originally had a Mercruiser 525 in it and now I got a 496 in it... Don't ask. Now on my dash I've got an oil temp gauge that does work because there is not hookup for it. My question is, is it possible to add an oil temp sender to the engine so the gauge works? if so is this going to be costly?

Easy and inexpensive to add the oil temp sensor. Take the remote oil filter base off the exhaust manifold mounting position and remove the filter. Drill the correct size hole and tap out an 1/8" pipe size thread up thru the bottom of the filter base unit between the center nipple and the edge of the outer flange that the oil filter itself seals to. Install an 1/8" oil temp sending unit up into this tapped hole and connect it to your dash gauge and you will be measuring the oil temp at the best point before it goes thru the oil cooler and back to the pan.

Best Regards,
Ray @ Raylar

and very necessary

Hooked an oil temp gauge up on my 454 the exact same way, works great.

Ken

Ray - just wanted to say thanks. Your method worked great. First time my oil temp gauges have worked. Much appreciated, thank you.