Correct Best Oil LT35

In a nutshell, the API ratings mean a 'better' oil, the higher the letter. By better it means more additives giving less sludge, less deposits and less wear. Generally there is no harm in going with a higher letter, with the possible exception of very old engines (pre-50s) with white metal bearings, which tend to suffer with the modern additive package used. (They require ZDDP, an additive which is no longer used as it harms catalytic converters).

Viscosity has no relation to the API rating, and neither to whether it is synthetic or not although it's easier to find synthetic oils in lower viscosities. If the manual gives a viscosity range for the ambient temperature you use the van in I would stick with that, and go with SF or higher. The current 'best' oil at the time was SG, so the higher grades simply did not exist back then.

Some people may recommend a higher viscosity on an old (worn) engine, and I might agree IF you were suffering from oil pressure problems. (Although this would be treating the symptoms rather than the cause).

The 40 part of the viscosity relates to the thickness when hot, so in terms of that they are all identical. The 5/10/15 part relates to the viscosity when cold, in which case the 5 grade oil will be thinner, yet still much thicker than it will be when hot so it would be fine to use it on that basis - i.e. the limiting factor is the hot viscosity, not the cold.

Hi.