Is 15W-40 ok to use on my 97 FZJ80 year around in Oregon weather, without damaging the engine? Very occasionally goes down to teens on winter time but it's usually maybe upper 20s - 40s... lately its been warm (for us) here. My 80 is not leaking oil but its consuming oil. Maybe a quart or less every 1000 miles. Been using 5W-40 syn oil. Mobil 1 TDT in the past currently I am using Rotella T6. I just want to try the 15W-40 on my next oil change but weather here is changing slowly to cold.