Multiweight for Fall

Well, I'm creeping up to the 50 hour mark for this season. Luckily the "season" has just started since the fall is when we really start hitting the water often. Anyhow, according to the previous owners, my boat "always" had straight 40wt oil, so I continued the tradition this year and fed her Valvoline 40wt racing oil this spring. I have 5 qts of Pennzoil 15-40 sitting on the shelf and I was thinking of throwing that in for the fall since we've been known to ski well into November. I know we've been over this time and time again but I'm paranoid about switching the weight on an engine for the first time ever if it has in fact only been fed straight 40 before. Thoughts? OH- and I don't want to damage the engine using the straight 40 in ~ 50 degree weather. Is that a myth? I heard of blowing rear seals, etc...?

I'm thinking around 50 degrees or so outside temp. It also takes a while for the engine to get up to temp in that chilly water too. We usually adhere fairly closely to the 100 degree rule (50 water/50 air). Here in Chicago we have to maximize the season as much as possible. We'll ski until the lakes freeze.

I run straight 40wt all year, we ski in 40 degree air temps about 45 degree water temps. I am running it in a 454, she cranks a little slow and is very cold blooded. She requires about a 3 minute warm up before ready to leave the dock and preform well.

Well guys, I just filled her up with straight 30 wt from Napa with a napa gold 1515 filter. The manual says
>=50 degrees: straight 40
< 50, straight 30.
< 32, 20 W 50 (multi)

He's got a 2000 Ski Nautique with the GT40 motor. I believe those oil recommendations in the manual will be the same for all 351w PCM motors.

79nautique wrote: By not using multi-weight oil your just putting a little more wear and tear on the motor during start ups using the straight 30 or 40 weights.

I thought about that, but since they don't reccommend a multi until below freezing temps, I have to wonder at what temperature the "multi" portion of multiweight actually kicks in. As in, at what temp do the agents in the multiweight oil actually alter the viscosity? At 50 degrees F, will my 15W40 actually be at a lower viscosity than straight 40, or will the agents in the multi not have "activated" at that temp?

So at 50 degrees, a 15W40 will pour like 15W oil and only becomes 40WT when the engine warms up?

Did some quick googling and found some info here:

An oil sold as 10w-40 is no thicker than 10 weight oil under Winter (10w) conditions, meaning below freezing. The 40 means it is no thinner than 40 weight oil at 212° Fahrenheit. So, the first number tells us the performance of the oil at or below the temperature of freezing water, and the second number tells us the performance at the temperature of boiling water. The chemicals added to the oil to accomplish this are called Viscosity Index Improvers (VIIs).

PCM Specifies 15W-40 for my '02 Excalibur. Aside from marine specific grades 15W-40 is most commonly a diesel grade, but is also rated for spark ignited engines On the back of the bottle the API seal has a rating something like CI/SL (the C in CI stands for Compresson Ignition and the S in SL stands for Spark ignited).

For simplicity I switched to 15-40 in the 351W in my '79 Nautique. Been using it for 2 seasons and it works great.