If only the formula is changed to provide the 5W to change to 15W.....then use the 15W as there is likely less "additives" to allow the oil to remain slippery and/or flow at lower temperatures. Less additives means there is more oil in the mix (This is one of the reasons they have started to recommend a straight weight low detergent oil for engine break in....there is more oil to lubricate and less additives to take up space).
There really isn't much of any significant difference in the viscosity of these two oils at temperatures above 50 degrees - the 5W vs. the 15W rating is made at ......so it isn't going to make much of a difference either way.
However - at this point we need to confirm the 15W version of T6 has the 1,200 ppm of ZDDP that is on both the 15W-40 T4 and the 5W-40 T6. The label states it is rated as MA/MA2 rating which approves it for use in motorcycles so it is likely wet clutch safe.
Direct comparison of the 5W-15W is more than just comparing the viscosity, as they also test the oil for the "cranking resistance" at the low temperature and the oil must meet both parameters.
https://wiki.anton-paar.com/en/sae-viscosity-grades/
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