OMD (oil mineral detergent) 80 is basically the same as OMD75 , just a newer version. I was MT cpl so know that either oil was suitable for use with the vehicles we had, from Land Rovers to Bedfords to Ferrets etc. If you are renewing the engine oil and don't have enough OMD80, use the civilian equivalent SAE30, a straight i.e. non-multigrade oil. I wouldn't mix the two. I own a ferret and all the advice i get from several sources is to use SAE30 mineral oil.