There is already an oil specifically made for his car -
Motul Nismo 15W-50. It's about $23 per liter online.
https://www.motul.com/en-US/products/46263?id=46263&product=NISMO+COMPETITION+OIL+2212E+15W-50
https://azupim01.motul.com/media/motulData/DO/base/nismo_competition_2212e_15w-50_en_fr_motul.pdf
TYPE OF USE
"
Specially developed for NISSAN 2.0L, 2.5L, 2.6L Turbocharged engines, SR20DET, RB20DET, RB25DET and RB26DETT with or without NOS (Nitrous Oxide System) fitted on NISSAN 180SX, 200SX, 240SX Silvia S13, S14, S15, Pulsar GTI-RN14, Bluebird U13, U14, NISMO 270R, Skyline 2000GTS-R, HR31, GT-R R32, GTR-R R33, GT-R R34, and requiring outstanding oil film resistance at very high temperature.
Engine producing medium to high fuel dilution in the oil.
Other applications: Racing car, Drifting car, Tuning car, Check with engine tuner before use."
RECOMMENDATIONS
"Oil change: 1865 Miles / 3000 Kms.
Do not mix with others lubricants.
This product is not covered by any NISSAN Motor Company warranty."
It made me think why are you even questioning the recommendation by the people who built your engine? ^This question is for them. If they say Motul Nismo 15W-50 is O.K. for the winter - it's done deal. Your car is not a regular car to worry about the cold stars, especially if it's stored indoors.