I have one S&W .38, it's old, and if it's too old to shoot modern ammo can anybody advise. Year of manufacture unknown.
If it's pre-model-numbered (i.e. there's no model number on the yoke frame), it's pre-1958 and .38Spl+P ammo is not recommended per the S&W "Revolvers - Modern Style" manual.
IIRC most knowledgeable folks recommend low-pressure loads for old .38S&W top-break revolvers to prevent premature wear. Reloading manuals often specify "Top Break" loads with less powder than the regular loads.
S&W made .38 caliber revolvers in several .38 calibers, 4 Hand Ejector frame sizes (I, J, K, & N), and a number of top-break configurations. An "old S&W .38" is too vague to get more specific about your particular gun. For a more comprehensive answer, please post some more info, including whether it's chambered in .38Spl, .38S&W, or .38 Long Colt.