Is it true that no prime larger than $241$ can be made by either acting or subtracting $2$ coprime numbers made up out of the prime factors $2,3,$ and $5?$
Update
Above example is clearly wrong, as shown by MJD. New question:Is it true that $251$ cannot be made by either acting or subtracting $2$ coprime numbers made up out of the prime factors $2,3,$ and $5?$
You mean like $162 + 625 = 787$?
Intuitively, it would very surprising if this conjecture were true. There is no reason to expect that the sum or difference of two arbitrary numbers would not in general be prime, and there are quite a lot of primes, so something surprising would have to happen for this large family of sums and differences to almost completely miss all the primes.
Brute-force computer search finds many counterexamples; for example $2^{19} + 3^4 =524369$. If you are interested in this kind of conjecture, learning a minimal amount of computer programming would be a good investment of your time.