Google tells me the MC4558 is the same circuit as the LM1458, with improved frequency bandwidth.
I've never tried the MC4558 in my Tims/Timmys but I can say that the LM1458 is cheap and readily available from most any electronics shop.
I don't think they're the same. The numbers give you a good indication what the circuit topology is and the letters suggest the manufacturer. The MC4558 is a circuit match for the classic TS808 JRC4558, not the LM1458.
Both good choices to try in these effects, but then you've just got to choose the one that sounds best to your ear in any particular effect. Oh and try the NE5532 too, my fave by far.
Some opamps sound identical, but certainly not all of them no matter what Visual Sound's opinion on it is, and the way they react to your setup can be noticeably different. I had an MI Audio Blue Boy Deluxe and couldn't have the Gain control over half way because my amp just choked and chords almost immediately decayed with no note sustain whatsoever. I tried a few different opamps and the NE5532 and TL072 were entirely different, no immediate choking and the whole gain range was useable.
It's not just mojo either, input impedance can differ widely from opamp to opamp, and as input impedance plays a big part in the filter created with the input cap it will logically and measureably change the frequency response.
I'm certainly not saying all opamps will be different, or that the difference will be more than just subtle in some effects, but Bob Weil's opinion about the matter is simply that, his opinion.
Opamps are not like changing the knobs on pedals.
They are IC's - Integrated Circuits, each chip is a collection of resistors, capacitors, transistors etc...in their own right, and thus imparting their own sonic characteristics upon the circuit as a whole... Of course opamps will sound different one to the next within a larger context of the whole pedal, and influence such factors as usable gain range, clipping and frequency output.
I swapped the stock op amp in the Earthquaker Devices White Light pedal for an LM-1458 (per their recommendation) for taming the brightness I was having with my setup. Worked great for me, no snake oil or wishful thinking, but a difference I could easily hear. Turned a pedal that I really liked into one I could really use without changing the character or openess of the pedal.
Op amps are cheap, try them out, just make sure the pin layout is the same as the one you are replacing. I found contacting the folks who build the pedal you may want to swap op amps in can steer you in the right direction of what to get to fine tune your sound.