Hi Maxy,
Some people here have different opinions than I'm going to give you. I'm not sure why ZaneO doesn't want you to switch to syn due to the condition of the seals, it shouldn't deteriorate them any more than they are, and if you're not leaking now, you shouldn't leak a synthetic of the same winter weight.
For that older engine, I wouldn't recommend using an oil with a
lower summer rating (the second half of the viscosity number), especially if you're using some oil at this point.
My recommendation would be Mobil 1 0W-30 for those cold WI winters. That will give you a 0-weight at startup, but the protection of a 30-weight when warmed up and during the summer. I can't find this locally anymore (used to get it at WalMart) but maybe they have it in your colder clime. You can also get it at
www.mscdirect.com, but it's more expensive there.
And yes, that's what I use. I have 2 GM's ('88 and '90) in my family with over 110,000 miles each that have been living on Mobil 1 5W or 0W-30 for their whole lives, and don't require any oil between changes or drip any on the garage floor.
And yes, for those of you who use different brands or weights, I am an idiot, I was raised by wolves, I'm un-American, and I suck, so you don't need to post back and tell me that.