Saving Old Seagulls

Since its now cold, the lakes will be icing up soon, maybe this weekend, and the water temp is cold, 32- 40F ( 0- 5C), would it make sense to use a multi-grade like 85W90? The manual suggests going from 140 SAE to 90 SAE or even "an engine oil in freezing conditions."

Are there any engine or gear box problems using multi-grade oil in near freezing conditions? The other hazards like freezing body parts are painfully obvious.