Oil viscosity question for the room

[ReBob]

I've got a '97 1845C with ~2,400 hours on it, I got it in '01 with <700 hours. Minor repairs since then (hoses mainly, and all the hydraulic ram seals fell apart in the last several years) except for rebuilding the injector pump. It still runs fine and I use it mostly for hauling firewood out of the forest and trying to keep roads scraped somewhat smooth. I've noticed that it seems like it doesn't have quite the hydraulic power it used to, lifting capacity, etc. but this is mostly just gut-feel and completely subjective, and seems most noticeable when it heats up. The manual says (and I've always used) 10W-30 motor oil with the HTO additive. A number of years ago I bought some old and well used maintenance manual material and I noticed that in one section it specified 10W-40 with HTO. A friend that operated a big Cat skid steer for forest mulching has a big auxiliary oil cooler on top of his rig and when I asked him he said that the aux cooler made a big difference in his perceived hydraulic power. I'm wondering if I should switch to the 10W-40 to see if that helps my power loss issue, but I'm concerned about voiding the warranty (just kidding). Any thoughts on this idea?