Not sure if you mean the prefilter or the main filter, since primary and secondary filter are confusing terms which can easily be mixed up...

My '78 never even had a prefilter when I bought it, it doesn't really do much IMO as the tank strainer mesh is the same size roughly as the prefilter. I have installed one since and it has only caught a tiny amount of crud over about 10,000 km.

Now the main filter will certainly benefit from an upgrade and I intend to one day. CAT 1R-0749 or similar is what I have in mind and has been done before.

A large filter (physical size not filtration microns) gives you more flow and much longer intervals between changes.

Have some doubt whether the filter size will help with gelling, maybe look into fuel additives instead?

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1978 300D, 373,000km 617.912, 711.113 5 speed, 7.5mm superpump, HX30W turbo...many, many years in the making....
1977 280> 300D - 500,000km+ (to be sold...)
1984 240TD>300TD 121,000 miles, *gone*
1977 250 parts car
1988 Toyota Corona 2.0D *gone*
1975 FJ45>HJ45
1981 200>240D (to be sold...)
1999 Hyundai Lantra 1.6 *gone*
1980s Lansing Bagnall FOER 5.2 Forklift (the Mk2 engine hoist)
2001 Holden Rodeo 4JB1T 2WD