Hey, ready to change the oil in my 96 Formula 303 with 502 mpi's. I want to put in Mercruiser Quicksilver 25W 40, but I see there is a regular and a synthetic blend. The synthetic blend is rated FC-W. Is there any advantage to using one or the other? I'm in SW Florida, so, boating season is still on and typically 70-80 degrees this time of year. The water temp is usually in the 60's & 70's now (not to rub it in with the northern readers, but just to know what the conditions that the boat is operating in).

i use the regular in most applications,i don,t really think syn blend is worth the money.for me full synthetic or regular.

Dino oil is still good to use, in my engines I use synthetic Mobil one 15W-50. Last time I got it was at Walmart for around $23.00 for a jug. Drives, Merc High Performance. A lot of god oils out there.

FWIW circa '96 7.4lx mpi's here with no leaks until I ran Valvoline VR1, now both leak. VR1 is full synthetic, should've kept with non-syn and added zinc as I had previously done.

yep,i have seen several high mileage engines develope oil leaks after switching to full synthetic.

Same engine (but 2000) . Used the regular 25/40 for the first year or two and then when the synth blend came available switched over to it. Considering going to the full synthetic 20/50 as at the Merc dealer there's not a whole lot of difference in the price between them . (believe it or not)
Without getting into the entire controversial oil argument they're all very good and you won't have any problem.

Never seen a motor damaged from any oil, only lack of maintenance

Being in FL I would just use straight weight no W needed.

I use Dino straight 40 in the 454, and Brad Penn straight 50 in the 509

VR1 is available in conventional ,is a fantastic oil, and comes with plenty of zinc/phosphorus and foam control agents (think higher reving hyd flat tappet and hyd roller) lifters

Grey bottle is conventional...black bottle synthetic....was up thru this spring anyway.