1991 nissan navara TD27 engine issues

[Crash bandicoot]

shove a litre of diesel in the oil and let it idle for 10 minutes then dump it(repeat as nessassary) Don't rev it.

I take it that it isn't turbo charged? use the search function here to see pics of how to increase fuel may help.

[ChurchurDan]

Get a compression test done, with sludgy oil and no air filter it is probably poked.
It should do a 100ks no problem they just take a bit of time getting there.

[Ryno]

Check the fuel filter, when I got my terrano it was gutless as, took out duel filter and found heaps of sludge etc. new one installed and goes like a bat out of hell..sort of

[hamboy]

Also check the valve clearances, the valves have a habit of wearing holes in the rockers