Your 1964 Model 36 was manufactured prior to the advent of +P ammo designations. Prior to +P there were several high-pressure loadings of .38 Special ammo (usually designated "HV" or .38-44) intended for use in the large frame revolvers (S&W 38-44, Outdoorsman, Colt New Service, etc), but not at all intended or recommended for small or medium frame revolvers.
Smith & Wesson continued to recommend against +P ammo in the J-frame revolvers for decades (well into the 1980's, IIRC). Only in more recent years has S&W upgraded its ratings of the J-frames to allow for use of +P ammo. In fact, even the lightweight (aluminum alloy and titanium alloy) J-frames are now offered in .357 magnum chambering.
Be that as it may, recent or current production guns and manufacturer's specifications do not necessarily relate to earlier production guns. Use of +P ammo in your 1964 M-36 would be something to be considered only at your own risk.
Further, as anyone having much experience with the J-frame snubbies will attest, the added effects of higher pressure ammo (recoil, muzzle blast, muzzle flash) can make effective marksmanship extremely difficult under any circumstances, and certainly under defensive conditions.
Another point to consider is the inherent inability of a 2" revolver to make sufficient use of the higher pressure ammunition to actually achieve any significant increase in ballistic performance. Many controlled tests have demonstrated pretty conclusively that the 2" revolvers cannot be counted on to achieve anywhere near the expected performance of high performance ammunition. Multiple tests of terminal performance of ammo fired from the snubbies have repeatedly shown that none of the touted high performance ammo (including hollow-points) will perform as expected consistently. Actual testing in ballistic gelatin media just about always results in disappointing results with high performance .38 Special ammo fired from 2" revolvers, with much less than 50% (and frequently over 90%) of recovered bullets displaying any significant expansion.
A final point to think about is the fixed sights of your M-36. Standard practice for many years has been to regulate fixed sights to "standard" ammo, which in .38 Special is usually 158-grain bullets. Lighter bullets can be driven to higher velocities, but this will invariably result in bullet impacts well below the point of aim. Fixed sight revolvers can be modified to correct this only by significantly changing the sights (on the M-36 this would require cutting, grinding, etc, to achieve, thus permanently altering the revolver for one specific type of ammo).
Having carried and used .38 Special revolvers for over 45 years my usual recommendation is to stick with standard velocity ammunition with bullets close to the "standard" 158-grain weight. In the M-36 I would recommend either the 158-grain semi-wadcutter or the 148-grain wadcutter loads. The SWC makes reloading easier and faster (with or without speedloaders) while the wadcutter loads make a formidable defensive bullet at close ranges (actually, I like wadcutters in the chambers and SWC's as back-up ammo). All factory SWC and WC loads feature swaged lead bullets (little or no adulteration of pure lead) which are as soft as possible, generally providing for upset and/or expansion when striking harder materials (such as bone), and limited overall penetration to minimize dangers of damage or injuries beyond the intended target.
Moderate recoil, minimal muzzle blast, minimal muzzle flash, and higher probability of shooting to point of aim. Coupled with reasonable training and continued practice, these factors should provide a good level of confidence in your M-36 for defensive use.
Now we can listen to everyone telling me how wrong I am.
