Difference between RF ferrite and normal ferrite
Ferrites come in a variety of flavors. The main groups are ferrites for


Filters (for signal filtering, e.g. in telecommunications)
Power Conversion
Interference suppression (EMI filters, not to be confused with signal filters)


Ferrites for signal filters need to have low losses and must have a defined temperature factor in order to make the LC combination work well over temperature. Also, they should be stable over time.

Ferrites for power conversion must have low losses and a high saturation value.

A ferrite core used for EMI filters is usually quite lossy but has high permeability, allowing for a high impedance in the desired frequency range.

Your oscillator would go into the first group (signal filtering), and if you substitute your ferrite with one salvaged from an EMI component, you might end up having fairly high losses, preventing your circuit to have a good-enough Q-factor.

Then, there's the pitfall of using an iron powder core by mistake. If varnished with epoxy, they look quite similar to ferrites, but will not work for RF signal applications (at all, in any reasonable way) due to the comparatively huge eddy current losses they have because of the fairly large iron particles they're made of.

Good sources for further reading are the introductory chapters of these two large data books:

Ferroxcube/Yageo Data Book (Soft Ferrites and Accessories)

TDK/Epcos Data Book (Ferrites and Accessories)

And, above all, the best book about ferrites I could find so far:

E.C. Snelling, Soft Ferrites, Properties and Applications, 1969

I came across this book because it's in the literature list on this great page about producing your wound components, worth spending some hours reading through. Quoting the site's author: "If you are using a ferrite core and this book doesn't have the answer then you are in trouble. Wide ranging both in theory and practice. The maths is intelligible and (bliss o' joy) uses SI units."