I just finished sewing some more pieces with one of my 66-18s. Not one bit of trouble with the heavy thread. I didn't even have to tweek the tension. I think the geometry of the tensions and thread path has an affect on the tension.
Wintersewer,
I'm a firm believer in nailing things together for keeps. I've seen other bedding come apart when the thread fails. So last year when I was but a beginning sewest I started piecing the smaller parts together with the heavy thread. Once started I continued using what I had already bought.
Oh, I don't do eBay at this time. Our account is messed up and we haven't had the time to get with the eBay people to fix it.
Also for things like thread I prefer to buy locally. That way if I need something I don't have to order it and wait for delivery.
Joe