Has anyone had any problems using spiral wound casings on their friction shifters? SIS cables and casing are great. But I can't see any real need for them on a friction shifter. My only problem is all the vintage spiral wound casing is 5mm.

Why is 5mm a problem? Yes, brake housing should work fine for friction shifting. You can get some 5-to-4mm step down ferrules.

I can't see where it would make a difference on friction shifting. It's not like you have to route it inside anything, or have to worry about flex in .1mm increments throwing your index off. One of my bikes uses the stainless spiral housing for the FD bend above the BB and the RD bend from the last stop. The other housing is all "normal." The housings are definitely large enough for brake cable, as well.

+1 I will do it on friction shifting, allows me to match the housing color with the brakes. Non-starter on indexed shifting.

I agree the newer brakes stuff, like from '87ish onward, work great on friction drive trains. Since the piece is so short sometimes it works with 6 or 7 speed index too, since housing compression is not as a big a deal on those systems.

First generation SIS systems from the 80s still used compression housing for the last segment before the RD, and it worked fine. This was also in the pre-brifter days when there was no additional housing along the way between the downtube shifter and the RD.

I once briftered up a bike to 8-sp STI, used the same white brake housing on the whole shebang. It worked fairly well. I'd definitely not try it again, because I know better, and as far as 9s and 10s, there are enough issues with adjustment to deal with, no sense in messing it up with housing.

My most recent build is 5-speed, anyway, and it's pretty ambivalent about whatever is used.

Spiral-wound housing works ok for any kind of shifting setup, up to the point where longer and multiple curved sections of housing are guiding the cable.

At that point, and especially as the shift increments (sprocket spacing) get smaller, the motion resolution between the shifter and derailer deteriorates noticeably and can make shifting difficult and make indexing cable adjustments hyper-critical.

Friction shifters, even where only a "shorty", 90-degree piece of housing is needed, will still actually benefit noticeably in terms of the shifter's ease of use by using compressionless housing, so any shifting setup will also benefit, especially as the housing ages and accumulates dirt particles along the cable's path.
This doesn't mean you have to use compressionless housing, I'm only saying that it works noticeably better than lined-but-spiral-wound housing.

Campagnolo's newer derailers have an actuation ratio that is a little bit closer to 1:1, so won't lose as much motion accuracy to "soft" housings as will a Shimano derailer, but the housing is still important since Campy's advantage here is very small.
Modern, higher-end mtb derailers have moved toward 1:1 actuation, but there is almost no reason to use spiral housing on these for the shifter cables.
As much as I've tried to retain vintage housings by fitting plastic liners in them, you can still see the housing distort when the shifter is moved, evidence that motion accuracy is definitely being lost to the housing.
And more importantly, the difference can be felt, especially when having to shift under the stresses of competitive riding, when shifts need to be completed fast, so the rider can return to an out-of-the-saddle assault on an uphill, undulating course.
This difference in shift performance between housing types is much like the difference in shift performance between the inferior ratcheting friction shifter and a true retrofriction (spring clutch) shifter, but these two shifter types only act differently during downshifts to a larger sprocket whereas spiral-wound housing affects shifting in both directions.

Lastly, note that modern shift housing needs to be used with ferrules designed for use with this kind of housing, otherwise A) the ends of the housing strands tend to migrate through the cable stop's tiny cable hole and wedge against the moving cable, because B) The ends of the housing strands cannot penetrate and settle into the ferrule's plastic cushioning, leaving the strands more unevenly compressed as the housing's curvature eventually puts the square-cut end of the housing askew.

Also, Varsities and certain other old frames used smaller-diameter wound shift housings and have cable stops that aren't big enough to accept ferrules, and even stepped ferrules will jut outward away from the frame tube unless the ferrule is machined flat along it's length, which can easily leave an open gap along the length of the ferrule.
So I usually just fit spiral-wound housing to the bb area of Varsities and the like, with the housing trimmed back a bit to allow today's thicker spiral housing to fit into the stops. This also makes flexing the housing into place between the housing stops at the bb a whole lot easier.

The good news here, for vintage-riding enthusiats, is that some of the shifting performance that is lost to older, non-slant-parallelogram derailers can be re-gained by using modern housing.
And, in most older Simplex and Shimano derailers, a bit more performance can be found by messing around with the cage pivot spring's tension so as to adjust the derailer's angle (to match the freewheel's size).

While it may be doable to use brake housing for derailleurs, never ever use SIS housing for brakes. Bad things can happen.

Thanks lot of great info going on. I'm with WRK101 on the colored casings.

Did anyone say colored casings?

Wow, that Taylor looks brand new.

Is that an IG chain with Sram link running on the Sontour-6 freewheel?

Looks like a good-shifting setup as well.

........this ^^^. Given the modest price, I cannot imagine why anyone would not treat
themselves to something similar to Jagwire new plastic lined housings and smooth stainless cables.

The performance advantages are easily noticeable. If I'm gonna spend the time and money
and energy overhauling a bike for my own use (which is all I ever do these days), then
the cables and housings all get replaced as part of the package.