Just changed the oil in my transfer box in my 300tdi 1997 Defender 90. I bought Millers EP 75w/80 GL5 mineral (API 4/5). I bought the oil years ago and only just had the time to change it along with all the other oils. Looking at the manual it states 75w/90. I realise it's a bit thinner but has anyone had any issues using this grade?